Category Archives: Electrical Topics

Harmonic Distortion of AC Power

Initial post: 6/7/202
Minor edits: 6/8/2020

I’m posting this here because it came up on a boating club Forum that I follow.  As I have said often, my “target audience” is people in boating that do not have much prior background in matters of electricity.  This topic is a bit arcane, and does tend to be an advanced topic.  But at the same time, it does show up as a symptom that affects some boaters in some situations, so I offer it here for awareness.

Here is the question that started the discussion:

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“I would like to elicit opinions from the electrically minded of us regarding the following.  When running my NL 9Kw gen at anchor my Dometic/Cruisair heat pumps (240V, 16000 btu) work fine with just one of my Magnum Energy MS2812 (2800W, 125A charger) active to charge the batteries. But, when the 2nd charger is activated (now balanced loads on the gen legs), the heat pump compressors stop active function (no heating/cooling), fan drops to minimum level, but, amp load is unchanged. The above occurs whether 1 or all 3 Dometic units are running (this is not about trying to start one of the compressor motors with the gen loaded).  I have not noted this interference when the battery charging load is minimal.  The gen amp output at 100% is 37.5/240V.  Max charger demand is 17A both legs.  All 3 heat pumps together draw 13-14A. There is no problem if the water heater is run (240V/10A) with the heat pumps on and just one charger (brief test – 40A on one leg).

“It seems as though there must be some type of electrical interference that is occurring when the 2nd charger is added to the circuit affecting the heat pump compressor motor function. Any ideas as to what this might be and how it can be tested for? Emails were sent to NL and Dometic with no response. Thanks!”

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Here is my response to this question, edited for completeness, which I offer to others who may be experiencing similar intermittent, “weird” symptoms:

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What you are describing sounds like a somewhat out-of-the-ordinary (but not “extraordinary”) problem called Harmonic Distortion.  Here’s the electrical theory of HD in four sentences: A pure resistance – water heater heating element, light bulb, running motor – draws current in linear proportion to its impedance (according to Ohm’s Law).  Electronic devices do not follow Ohm’s Law;  they can and do draw current in short bursts within the AC sine wave voltage cycle.  These electronic devices are called “non-linear” loads.  Since in non-linear loads, current does not follow Ohm’s Law against voltage, the apparent internal impedance of the source can cause the waveshape of the AC voltage to distort (dip, flatten at the top and bottom), rather than be or remain a pure sine wave, clean as the driven snow.

So in your situation, the inverters are AC loads being used for battery charging, but the battery charger’s internal DC circuits are non-linear, “switch-mode” devices.  That creates non-linear current demand on the input AC waveform that is reflected back into the source.  The system doesn’t fail on shore power because the apparent impedance of the shore power source is many, many, many times less than the apparent impedance of the genset.  That doesn’t mean the phenomena isn’t there on shore power.  It just means the source is big enough to overcome the magnitude of the non-linear load component.  On shore power, the ratio of load impedance to source impedance is sort of analogous to David-on-Goliath.   But with the much smaller capacity of the genset, the aggregate effect of the switch-mode current demand can affect the shape of the genset’s output voltage sine wave.  Here, the ratio of load impedance to source impedance is definitely David-on-David.  What tends to happen with Harmonic Distortion is that the positive and negative peaks of the AC sine wave flatten, although more complex distortion is possible in extreme cases, even to the point of approaching a square wave with a flat top and very low peak voltage.

You mentioned in your post that you have a 9kW NL genset.  Nine kilowatts is somewhat under-sized for a 250V, 50A boat.  The power that can be absorbed by a 240V, 50A load is 12000 Watts, or 12 kW.  What you have is NOT “bad” from the perspective of genset loading or the perspective that you rarely need the entire capacity of the generator anyway.  But, if what you have is a symptom related to Harmonic Distortion, the smaller genset will have a higher apparent impedance than a larger genset would have.  The higher the apparent impedance of the source, the more likely it is that Harmonic Distortion would present itself as a noticeable and annoying symptom.

My conjecture that this is Harmonic Distortion is easily confirmed with an oscilloscope.  In the old days, that was the only way to see it.  But today, you can confirm it easily it if you have a means to read TRUE RMS voltage and a means to measure the TRUE PEAK voltage.  The peak of a 60Hz sine wave should be 1.414 times the RMS value.  I use an Ideal SureTest 61-164 or 61-165 circuit tester for this task.

So let’s assume you have a stable 60Hz voltage at 118V when running on the genset.  And we must also assume you have a stable 60hZ frequency, ±2 hZ, when running on the generator.  Multiply the 118 x 1.414, and the peak of the voltage waveform should be 167V.  If you then measure the actual peak, and it’s – let’s say – 156V, then you know you have Harmonic Distortion taking place, and the wave form isn’t a pure sine wave.

Now, the tolerance of the inverter/charger(s), the SMX Controller electronics and the blower drive electronics of the heat pump to AC voltage waveform shape, for which they, themselves, are responsible for distorting in the first place, may not be favorable.  That is a vicious circle.  It’s creating something that it, itself, can’t live with.  Since the genset is also feeding the Dometic SMX heat pump control unit and the blower and compressor control electronics of the heat pumps, those circuit boards can also be impacted by distortion of the voltage waveform.  Symptoms across the onboard system can be unpredictable, and can vary from attachment to attachment.  Pure resistance loads will not be affected, but electronic devices can be to varying extents.

Harmonic Distortion and Power Factor are two of the most challenging problems power utility companies have to manage.  A distorted AC voltage sine wave waveform is called “dirty power,” and it costs utilities a lot of money to manage.  Buildings with banks of computers and servers cause huge HD problems on the power grid, often affecting their neighbors and neighborhood.  Virtually all electronic devices cause Harmonic Distortion, right down to the family flat screen TV and stereo.  Power quality is a huge problem at the level of commercial power utilities serving residential neighborhoods.

And by the way, from the perspective of the 9kW NL generator itself, the higher apparent impedance and distorted wave shape will cause additional heat in the windings of the genset.  That heat is not related to useful work done by the generated power.  It amounts to excessive waste heat of which the genset’s cooling system has to dispose.  This can be worse than having unbalanced 120V loads on each side of the genset.

The fix?  You’d need a bigger capacity generator; i.e., one with lesser internal impedance.  With a lower reflected impedance, the genset would maintain the shape of the waveform for equivalent non-linear loads.  Or, your can just choose to live with it…

I have not written about Harmonic Distortion or Power Factor for my website because it’s definitely not a beginner’s/layman’s topic.  (Well, I have now, haven’t I?)  And even if you have HD, there’s little that can be practically done.  But if you want to read more about HD, click here for a fairly readable and reasonably good explanation from Pacific Gas & Electric; and click here for a better explanation of non-linear loads.  Start on page 3, at the heading called “ELECTRICAL HARMONICS.”  Skip the math; you don’t need it to understand the concepts.

Hope this helps.  And of course, this is only a guess on my part…   Cough, cough, choke, choke…

I wish I could recommend something practical that would make this better, but in the current system configuration, I think it’s a permanent restriction.

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Understanding Harmonic Distortion is complex and it’s definitely an advanced problem in an electrical distribution system.  What I’ve written above is just the very tip of the the technical iceberg.  But, although relatively rare, HD can produce observable symptoms related to the performance of boat AC electrical attachments.  It can affect the quality of sound from an entertainment system or produce what looks like interference (snow, lines) on a TV.  And, it can affect the operation of other types of equipment, like network routers, DVRs and printers.  If you have these symptoms and all else has been ruled out, consider Harmonic Distortion as a possible cause.  If you have these symptoms, it will be necessary to call in a skilled professional electrical technician to troubleshoot and confirm the diagnosis.  The tools that are necessary are expensive, and the skills to appreciate and understand the causes are advanced.  This is not a job for a residential electrician.

ABYC Electrical Standard Mapped to Sanctuary’s AC System

4/20/2020: Significant editorial updates to content.
5/27/2020: Added borders to images via HTML edits.


All boaters at one time or another get involved in discussions about what boats “are required by standards and codes to have or to do.” This comes up every time the owner is faced with getting a boat survey. A boat survey report usually makes copious references to “ABYC Standards” and to “industry best practice.” But the vast majority of boat owners do not work in a world of industrial codes and standards and are not familiar with what they are, what they are intended to do, and how they are used throughout the marine and commercial business world (especially, the insurance risk world).

This article is in the form of a stand-up classroom presentation. Slides are presented along with text (“speaker notes”) that describes the slide’s content. This is a mix of “engineering” and “safety.” My hope is that this material will make sense in this format. What I do in this article is look at the “electrical system” of our own boat, and compare that to the requirements of the principle ABYC electrical standard, E11, “AC and DC Electrical Systems for Boats.”

Our trawler, Sanctuary, is a Monk36 Trawler fit with two 120V, 30A shore power service cords. In our case, the shore power cords are configured so that one feeds the house AC loads and the other feeds our heat pump AC loads. Many boats are configured in the same way, but other configurations are possible. Our house loads include a battery charger for our genset start battery, fridge, hot water heater, inverter/charger and several utility outlets. The heat pump loads include one 5kBTU self-contained unit and one 16kBTU self-contained unit and a raw water circulator pump.

While configurations of individual boat electrical systems may be different, the ABYC Electrical Standard E11, “AC and DC Electrical Systems on Boats,” applies equally to all electrical system configurations on all boats of all designs and hull forms. Boats that adhere to the ABYC electrical standard are highly likely to be safe and compatible with 2020 shore-side infrastructure (marinas, boatyards, community, condo, municipal and residential docks). These standards are intended to maximize the safety of the boat; safety from shock hazards, freedom from ground faults, freedom from accidental fire hazards and much worse. I strongly encourage boaters to bring their boats into compliance if that is not already done!


Figure 1 shows an “energy flow diagram” of the total electrical system of a typical cruising boat, comprised of three separate divisions. The central electrical system is the vessel’s DC division (shown in red). This is the division that starts the engine and powers navigation lights, pumps, windlass and miscellaneous navigation equipment. All engine-powered boats have DC systems, but AC divisions are optional. Sanctuary’s platform also has an AC division (shown in green) which allows captain and crew to enjoy the comforts of a shore-side residence. Interfacing between the DC and AC divisions is a means to charge the batteries, and optionally, also use the batteries to power all or part of the AC division.


Note: in this topology view, solar battery charging systems would be part of the DC Division.

Note: out-of-scope for this article is the Bonding System Division of the electrical system. Those interested are referred to my article “Bonding System Design and Evaluation.”

Figure 2 shows the interfacing division with an inverter/charger instead of a battery charger. The red highlighted lines show the Inverter/charger in “Invert” mode. For the inverter to be in “Invert” mode, no other AC power source is available to the vessel; ie, no shore power, and no onboard generator running. Absent a source of AC power, the inverter draws DC power from the batteries, converts it to AC, and provides AC power to a subset of AC circuits on the boat. This operating mode would be the typical operating mode for boats at anchor, or boats underway on a travel day. While at anchor or underway, power is available for an AC coffee maker, a microwave, a crockpot, AC space lighting and entertainment systems, and an AC charging source for computers, onboard routers, smart phones and tablet computers. At least, that’s what we do aboard Sanctuary.


In Figure 3, the red highlighted lines show the flow of AC power when the boat is connected to shore power via a dock-side pedestal. AC Power enters the boat at the SHORE POWER INLET, passes through a MAIN DISCONNECT BREAKER to, and through, the GENERATOR TRANSFER SWITCH and on to a DISTRIBUTION PANEL which supplies HOUSE LOADS. AC Shore Power passively “Passes Through” the INVERTER/CHARGER to power a subset of AC loads, and the inverter/charger device acts as a DC BATTERY CHARGER.

Note: in this topology view, the inverter/charger is fully integrated into the boat’s electrical system, and automatically switches between “Standby/Pass Through” mode and “Invert” mode as AC power from another source comes and goes. If a boater in a neighboring slip accidentally turns off Sanctuary’s pedestal breaker(s), our inverter/charger automatically transfers to “Invert” mode to maintain AC power to it’s attached loads. This configuration is the ONLY use case that ABYC supports for inverters or inverter/chargers installed aboard boats.

Figure 4 shows the above AC Electrical System components mapped to the actual wiring diagram detail of Sanctuary’s installed AC electrical system. The remainder of this article focuses on ABYC requirements of the E11 standard related to the AC Division of the boat platform.


Note: Sanctuary is not fit with an Isolation/Polarization transformer (shore power transformer). Shore power transformers have a number of unique ABYC requirements and considerations. Consult the E11 standard for the treatment of these devices.

Note: I occasionally hear that an isolation transformer has been recommended as a means of avoiding the need to “spend unnecessary money” in order to fix/correct conditions aboard a boat that cause dock-side ground fault sensors to trip AC shore power “off.” I strongly discourage that thinking. The conditions that cause ground fault sensors to trip are often serious, potentially dangerous electrical safety or fire hazards. Transformers do mask safety problems which can be a threat to the boat and its occupants, but they DO NOT CORRECT THE UNDERLYING ELECTRICAL FAULT-CAUSING CONDITIONS.

Figure 5 is a clear view of the wiring detail of our AC electrical system. Notice that the neutral buss for house circuits has been divided so that the circuits fed from the Inverter/charger are separated from the house circuits that are not. Further, except as necessary for explanation, AC safety ground wiring is not shown on this diagram; that is a conscious choice made in the interest of simplifying the diagram.



The ABYC electrical standard is quite extensive and complex. This presentation only covers the major highlights that apply to the AC system division. Similar requirements apply to the DC division. Get these basics right and the boat will be well on its way to being safe. This presentation does not include a discussion of the requirements of onboard 120V load circuits; it focuses on the power distribution components of the AC division, to which we normally give little specific consideration.


By far the most common 120V, 30A shore power connectors are National Electrical Manufacturers Association (NEMA) L5-30R and L5-30P pairs. These are found on the familiar 120V, 30A commercial cordsets. I do not like them because I feel they are not nearly robust enough for the repetitive removal and replacement to which shore power cords are subjected in normal use. NEMA L5-30 connectors were designed 80 years ago for light industrial applications where outlets were sometimes ceiling mounted and machinery cords hung from ceiling receptacles. They were plugged in and given a twist, and they were rarely touched again. They are not intended to be roughly handled by boat owners and dock assistants, dropped on docks, stepped-on, rained-on, snowed-in and otherwise abused in routine service.

Which brings up an important point about all ABYC standards. The “requirements” stated in ABYC E11 are MINIMUM PERFORMANCE REQUIREMENTS. They do not require a particular piece of equipment or a particular manufacturer’s product. They simply specify minimum compliance requirements. So, NEMA L5-30P/R connectors ARE NOT “required” by the standard. What is required is a “grounding plug that locks into place” so it can’t “fall apart.” Also realize, ABYC standards apply to boat manufacturers, marine equipment manufacturers, and service technicians. Only indirectly do they apply to boat owners. The standards DO NOT contemplate that DIY electrical work will be done by owners, but they do contemplate that all work done by anyone will comply with the requirements.

0DA35F75-5CF5-4F08-B340-40F94896936A_1_201_aI have personally chosen to replace the OEM NEMA L5-30P shore power inlet receptacles with those made by SmartPlug, LLC ( (no personal financial interest; just a very happy customer). I personally feel SmartPlugs are much safer and more robust than L5-30 twistlocks, and they meet all NEC (UL, cUL, eTL) and ABYC requirements. That said, the SmartPlugs EXCEED the minimum performance requirements of the E11 standard.

The following slide shows requirements for the shore power CORDS and the shore power INLETS of the boat. The E11 standard refers to the “Type” of the wire. The cord’s “Type” descriptor is part of the information printed on (or molded into) the cord’s insulation, and should be easily readable on all marine-complaint cordsets. Don’t worry about the “Type” descriptor on Shore Power cables unless for some reason (I discourage this) doing a DIY shore power cord fabrication project. Simply buy products made by marine manufacturers and certified for marine use. The cordset manufacturers will have covered all that’s necessary for ABYC standards compliance.


The following slide illustrates a very important concept for shore power systems which all boaters should know; most especially, those who do DIY electrical projects!  At the head of the dock, in the facility’s electrical service infrastructure, the safety ground conductor is bonded (connected) to the neutral conductor. This is an NEC code requirement for all sources of AC power throughout North America, and results in a system referred to as a “Grounded Neutral System.” In a “Grounded Neutral System,” the neutral is intended to carry all of the current returning from the boat to the shore-side source. By design, the ground conductor IS NOT intended to carry current except to trip a circuit breaker in a fault situation. Thus, the neutral-to-ground bond is located in the facility’s infrastructure for both 120V and 240V systems.


The following slide emphasizes the boat-side of the shore power connection. The E11 standard requires that there be no neutral-to-ground bond(s) on the boat. At this point, for clarity, that firm statement can be modified to read, “there must be no neutral-to-ground bond(s) on the boat when operating on shore power.” The reason for this distinction now will become clear later, but for shore power, if there is a neutral-to-ground bond on the boat, that wrongly-placed bond creates a connection between the neutral conductor and the ground conductor that electrically parallels the two conductors all the way back to the dock-side infrastructure’s correct ground bond. Since the ground conductor on the boat is in direct contact with the sea water in which the boat is floating, this also parallels-in a ground path through the sea water. When all of these paths are in parallel, current that should flow only on the neutral will divide and flow in equal amounts on both conductors, and in some amount, through the water itself. By definition, this is a “ground fault,” and it will trip power “off” if there are ground fault sensors on the dock-side pedestal, but it can also kill people, pets and wildlife in the water. Incorrect neutral-to-ground bonds on boats are a primary cause for AC power leaking into the water, and can lead to incidents of ELECTRIC SHOCK DROWNING. For further information, readers are referred to my article on “Electric Shock Drowning.”


The following slide shows correct and incorrect wiring examples. In my article entitled “AC Electricity Fundamentals – Part 1,” I explain that a boat connected to a pedestal is intended to be wired like a sub-panel in a residential installation. Many residential electricians and DIY boat owners do not understand that technical detail, and so often connect neutrals and grounds together as they would in the main panel of a residence. On boats, as explained above, this is WRONG and DANGEROUS. Those who DIY must understand this natty technical detail.


The following slide shows the next major component in the flow of AC power into the boat: the Shore Power MAIN DISCONNECT BREAKER. This device is mainly for overload (and since 2012, ground fault) protection. Note that for 120V, 30A circuits, both the hot conductor and the neutral conductor must be switched, so this disconnect must be a 30A, “double-pole” circuit breaker with either a single operator handle or operator handles that are mechanically interconnected so if one side trips, the other side is also opened.


Boats built before 2012 will not have OEM ELCI (Equipment Leakage Circuit Interrupter) circuit breakers installed. That is OK. Although required since 2012 on new construction boats, ABYC states that boats that complied with the version of E11 that was in effect at the time the boat was built by the OEM manufacturer are “grandfathered” for compliance. Note that MANY MARINE SURVEYORS do not choose to adhere to/acknowledge the ABYC “grandfathering” policy. That can result in an inappropriate non-compliance finding in a boat survey.

The following slide shows the MAIN DISCONNECT SWITCH on a boat fit with 240V, 50A service.  The significant difference is that here, only the two hot conductors (L1 and L2) are switched. The neutral is not switched. Thus, a double-pole breaker rated at 50A is appropriate here. As before, this breaker must have either a single operator handle or operator handles that are mechanically interconnected so if one side trips, the other also opens.


Note that the neutral-to-ground bond is only correctly located in the shore power infrastructure, which is one of the National Electric Code (NEC) “rules” for residential and light commercial 120V/208V/240V electric services.

The following slide illustrates another very important wiring detail. Recall, Sanctuary is served by two 120V, 30A circuits. Earlier, we saw that neutrals and grounds MUST NOT be connected together aboard the boat. This is a similar case, and for the same reason. Here, it’s essential that the neutrals from Shore Power Circuit 1 and the neutrals from Shore Power Circuit 2 be SEPARATED aboard the boat. The reason is, both of the neutrals run back into the marina pedestal, or may run all the way back to the marina main service panel. If they are connected together on the boat, they become electrically paralleled all the way back to wherever they are ultimately joined together (pedestal junction, panel neutral buss, etc). All current returning from the boat will divide and flow equally on both neutrals. By definition, that is a “ground fault” at the pedestal circuit breakers, which will trip both breakers and interrupt power to the boat. But even more importantly, if one of the shore power cord neutral conductors were to fail open (due to, for example, a burned blade on a NEMA L5-30P twistlock plug), the other neutral circuit would become overloaded and could easily become a fire hazard aboard the boat. Preventing that fire hazard is why understanding and complying to these standards is important.




And by the way, the “wrong way” is a common way to find neutral wiring done on older boats.

Check your boat.

The following slide highlights the need for Equipment Leakage Circuit Interrupter (ELCI) devices for protecting against ground faults on the boat.


Those interested can read more about ELCI circuit breakers in my article entitled “ELCI Primer.”

The ELCI requirement was added to ABYC E11 in 2012 for new boats. ELCI devices are intended to both protect from overloads and detect ground faults. Ground faults on boats can result in dangerous levels of AC power being dumped into the water, which is a hazard that can lead to Electric Shock Drowning (ESD), as discussed previously.

An ELCI device on the boat is the same thing as a “ground fault sensor” on the dock-side pedestal (ground fault sensors on docks have many acronyms, including “EPD,” “GFD,” “GPD,” and “RCD;” don’t worry about what they’re called. By any name, they do the same thing.) ELCI devices also do the same thing as pedestal sensors, but the ELCI is physically installed aboard the boat. The value of having an ELCI on the boat is twofold. First, the simple act of installing an ELCI will flush out any silent, hidden wiring problems that currently exist on the boat. Second, ELCI will trip instantly upon the spontaneous emergence of a ground fault issue on the boat at some later date, so the boat owner will become aware of it, and be able to initiate repairs, as soon as it surfaces as a safety issue.

The following slide introduces the concept of a GALVANIC ISOLATOR. Galvanic Isolators are very important to controlling corrosion of underwater metals on any boat.


Galvanic Isolators are installed IN SERIES WITH the safety ground conductor AT THE POINT WHERE THE GROUND CONDUCTOR ENTERS/EXITS THE BOAT. Nothing – NOTHING – should be connected to the side of the isolator that leads to the shore power inlet connection except the actual safety ground conductor, itself.


The E11 standard considers Galvanic Isolators to be “optional” equipment, but if they are installed, the standard provides installation requirements.

If a Galvanic Isolator is NOT installed, the rest of the GROUNDING CONNECTIONS are still mandatory.


Earlier, above, the ABYC requirement that “there must be no neutral-to-ground bond on the boat when connected to shore power;” was mentioned with the proviso that it would “become clear later.” Now is the time to clarify as we look at the topic of POWER-SOURCE SWITCHING. The following slide shows the three possible sources of AC power on Sanctuary: 1) shore power, 2) genset, and 3) Inverter. The North American design standard for ALL AC power sources is, ALL power source neutrals are grounded at the source. Since shore power sources are grounded on land in the facility infrastructure and NOT aboard the boat, and since both the generator and the inverter are located aboard the boat, then how is it possible for them to be “grounded at the source” if neutral-to-ground connections are not allowed on the boat? Well, compliance is accomplished through appropriate source transfer switching.


Note the construction of the GENERATOR TRANSFER SWITCH shown on this slide.  That Generator Transfer Switch on Sanctuary is a three position rotary switch: “Shore,” “Off,” “Generator.” When the switch is in the “Shore” position, the generator’s neutral-to-ground bond is switched out of the circuit, thus meeting the shore power separation requirement. When the switch is in the “Generator” position, the shore power circuit is switched out of the boat’s electrical platform, thus permitting the onboard neutral-to-ground bond at the generator. The same type of logical switching is accomplished for the inverter by a relay located within the inverter.

Note: ABYC A31 requires that Inverters installed on boats be certified to UL458 (Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts) to ensure this grounding management relay is present. ABYC E11 includes ABYC A31, amongst other boat electrical standards. BOAT OWNERS SHOULD ENSURE THAT ANY INVERTER INSTALLED ON A BOAT IS COMPLIANT WITH UL458. Especially, be aware that inverters from Harbor Freight and other discount sources will not be compliant to UL458 and are not suitable for use on mobile platforms like boats and RVs.

Following is a close-up of Sanctuary’s Generator Transfer Switch. This is a three-position rotary switch. There are other switching styles that use lockout slide mechanisms to accomplish the same thing. Here, the breaking of the neutral conductors is highlighted by the red ellipses.


The following slide is just a reminder of what we looked at earlier WITH RESPECT TO SHORE POWER SOURCES. For Shore Power, the neutral-to-ground bond is in the shore power infrastructure and NEVER on the boat.


And this slide shows the generator neutral-to-ground bond that is switched into the boat’s onboard AC system when the genset is running…


And this slide shows requirements specific to inverters…


The following slide moves to another very important safety issue. Rarely, it is possible to encounter a 120V dock power pedestal source in which the black (hot) and white (neutral) wires (or red and white) are physically reversed inside the pedestal or other location in the dock-side infrastructure. No, it should not happen. Yes, it should be found by the installing electrician before the circuit is put in service. But folks, it does happen (rarely, thankfully). I have seen it three times in 16 years of cruising.


What’s particularly bad about a “reverse polarity” situation is that it can be present and also be entirely symptomless on the boat. Electrical equipment aboard the boat will work normally. But, touch potential shock hazards are likely. Because this condition is largely symptomless, it’s important to detect it and warn the boat operator of the potential life-safety issue. The “RP” warning lights (and/or audible alarms) are connected between the Safety Ground (green) and the Neutral (white) conductors on the boat. There should never normally be more than a volt or two between those conductors. Anyone who sees a ”Reverse Polarity” warning light(s) illuminated on their boat should immediately DISCONNECT (physically unplug) the shore power cord from the pedestal and report the condition to facility management. This can be a potentially lethal condition in the right (wrong!) circumstances.


This slide shows “Reverse Polarity” warning lights wired between the safety ground and neutral conductors aboard Sanctuary.

Actually, Sanctuary has some duplication here. Our Generator Transfer Switch has Reverse Polarity indicators, as do both shore power distribution panels.
ABYC specifies the minimum impedance of RP detection devices must be ≥ 25kΩ. Since these devices are connected between the neutral and the safety ground, they are a possible path for small “ground fault” currents, and properly installed sensors on some boats can cause false trips of a dock-side or ELCI ground fault sensing device. This would be caused by either multiple sensors in combination or older incandescent sensors having too low an impedance, thus allowing too high a “ground fault leakage current.”

The most current revision of the ABYC E11 Standard (July, 2018) as of this writing (April, 2020) is 67 pages of “shall” and “shall not” requirements, technical tables and example electrical drawings. Far more than I have covered here. Furthermore, there are several other ABYC Standards that apply to electrical subjects, such as A27, “Alternating Current Generators,” A28, “Galvanic Isolators,” A31, “Battery Chargers and Inverters,” E2, “Cathodic Protection” and E10, “Storage Batteries.”

These standards – and all ABYC Standards – make us all safer. They save property damage losses and they save lives. A marina fire is one of the most terrifying things any boater can ever experience, and there have been several this year alone (winter, 2019-20). When we aboard Sanctuary arrive at a marina, we must assume all of the boats that will be our new dock neighbors are safe. All of those boaters must also assume that we are safe. These standards are the reason we can all have some confidence in those forced assumptions. If there are condition(s) aboard your boat that you know need to come into compliance, please do so. The family you save may be your own!

For the record, I’m not much of a fan of covered slips, either. Those roofs help with UV damage and weather, but in a fire, heat arising from the fire’s origin is contained by the cover and spreads linearly along the dock until the cover finally burns through. This greatly foreshortens escape time; and, not a good thing for survivability of boats that were otherwise uninvolved in the first place. Always think fire safety and escape routes…

Electrical Behavior of a 208V/240V Boat

This article discusses the electrical behavior of the two 120V AC circuits on a boat that is natively wired for 125V/250V, 50A shore power service.  Topics include current flow (Amps) in the different appliance loads, power limitations when connected through a “Smart Splitter,” and the constraints and limitations encountered with the use of certain shore power transformers when powered from 208V dock utility voltages.

Use Case 1: a boat wired with a 125V/250V, 50A shore power cord, but not fit with 240V appliance loads.

Figure 1 is a generic wiring diagram illustrating this use case.  The system includes a genset and a Galvanic Isolator.  In Figure 1, the dock power source is on the far left and the boat’s appliance loads are on the far right.  Dockside 50A circuit breakers are omitted for simplicity.  The 50A shore power cord is highlighted in the red oval.  One 120V load (the heat pump) is highlighted in red.  Other 120V loads (house loads) are shown in black.  This boat DOES NOT have 240V loads.  This use case is a very common “50A” boat configuration.

Use Case 2: a boat wired with a 125V/250V, 50A shore power cord adapted to two 120V, 30A pedestal outlets to obtain limited 208V/240V power.

Figure 2 is a generic wiring diagram illustrating this use case.  Most commonly, a “Smart Wye” splitter adapter is used (ref: Appendix 1).  A “Smart Wye” splitter has two 30A twistlock plugs (NEMA L5-30P) and one 50A receptacle (NEMA SS2).  The two 30A receptacles (NEMA L5-30R) are on the dock pedestal.  The splitter and the 3-pole, 4-wire, 50A power cord are shown in the red ovals.  The rest of this system is identical to Figure 1.

Figure 3 applies to both Use Case 1 and Use Case 2 configurations.  Figure 3 shows logical blocks instead of actual circuit detail in order to make it easier to visualize the electrical behavior in this AC system.  In Figure 3, incoming power is shown as being derived from “any suitable 240V source.”  Electrically, we really don’t care how we get shore power as long as it’s “3-pole, 4-wire” of the right voltages.  In Figures 1 and 2, the loads were shown as they are wired, but Figure 3 shows them as they are logically arranged in the overall electrical circuit.  As the drawing shows, the red-highlighted 125V, L2 heat pump load is connected in series with the black-highlighted 125V, L1 appliance loads.  These two load groups share a common “Neutral” conductor.  The Neutral conductor anchors and maintains the midpoint voltage of the series connection under varying demand conditions.

Visualizing this electrical configuration in the mind’s eye as two 120V loads connected in series across a 240V source is the first key concept in this article.

Having identified the electrical arrangement if the two 120V appliance load groups of this 240V system, further analysis is on a) the voltages present, b) current flows, and c) power available to do work.

Figure 4 shows the two series load components of this boat’s 240V boat system, each with 120V across them.  The L2 load group is comprised of the boat’s heat pump(s) and raw water circulator.  The L1 load group is comprised of the hot water heater, fridge, battery charger(s) and multiple utility outlets.  Measuring across the L2 load between points A and B, there are 120V.  Measuring across the L1 load between points B and C, there are 120V.  The series pair receive the 240V mains supply voltage measured between points A and C.

Next, consider the electrical currents (measured in Amps) flowing through the two series load groups in a variety of specific but different load circumstances.  Understand that in the following analyses, different specific devices are “on” and others are “off” at any specific point in time.  Assume the following scenario: the boat’s owners have been away from their boat for a mid-summer week.  Upon late day arrival at the boat, outside air temperatures are in the mid-to-high 80s with 85% relative humidity.  Our boat owners will turn on some space lighting, and will immediately turn on the heat pump for air conditioning.  They will turn on the hot water heater and battery charger, stow fresh veggies, ice cream and adult beverages into the fridge, and perhaps turn on the DVR/TV.

Electrically, assume the heat pump draws 20A.  Also assume that house loads (hot water heater, battery charger, fridge, space lighting, computers and DVR/TV) add up to drawing 20A.

In Figure 5, the heavy red line represents this 20A flow of current (Amps).  This example is a special case called a “balanced-load” condition; that is, both of the 120V loads just happen to draw the same amount of current (20A).  The Amps flow from the dock pedestal into the loads on one of the energized line legs (L1), and flow back to the pedestal on the other energized line leg (L2).  In this balanced-load condition, no current flows in the neutral conductor (N).

Very importantly, notice that no more than 20A is flowing anywhere in this system. A double-pole 30A circuit breaker that serves the boat via a Smart Splitter at the dock pedestal sees 20A on both legs, L1 and L2.  Since there is no place in the system carrying more than 20A, the 30A pedestal circuit breaker is perfectly happy.  The second extremely key concept to take from this article is that the 20A flowing to power the heat pump circuit is the same 20A that flows through the House circuits to power the water heater, battery charger, fridge and utility outlets.

The word “power” is highlighted above to make the point that the same 20A flowing in the two 120V loads does useful work in both 120V load groups.  The basic formula for “Power” is P = Volts x Amps.  So in the heat pump load group, we have 120V * 20A = 2400 Watts.  In the house appliance load group, we also have 120V * 20A = 2400 Watts of power doing useful work.  In total, we have 4800 Watts of work being done at this time, in this system.

Up to 30A is available from a 30A shore power pedestal without exceeding the capacity of the circuit breakers.  The maximum power possible for each load is 120 * 30 = 3600 Watts.  Because the two load groups are in series, the maximum work that can be done by 30A, in total, is 7200 Watts.  If the boat had access to its design maximum of 125V/240V, 50A shore power, there would be the potential for 240 * 50 = 12000 Watts, total.  It quickly becomes clear why careful load management is necessary when running with two 30A cords feeding a 50A boat through a 30A Smart Splitter.

Following from our earlier scenario, after an hour or so, the hot water heater has done its water heating work, the fridge has done its cooling/freezing work, and the batteries are fully charged.  But, the heat pumps are still running to cool the boat.  Now, although we have 20A flowing in the heat pump load, current on the house side has dropped to 4A for the DVR/TV and space lighting.  Figure 6 shows what happens electrically.

The heavy red line represents the 20A needed by the heat pump.  But this time, there are only 4A needed by the house, represented by the thin red line continuing through the House circuit.  There is no longer a balanced-load.  The arithmetic difference between the heat pump demand and the house demand is 16A.  That 16A returns to the pedestal in the system’s neutral (N) conductor.  In this example, as before, there are 120 * 20 = 2400 Watts of work being done in the heat pump load group, and 120 * 4 = 480 Watts of work being done in the House load group.  There are never more than 20A flowing in any part of this system.  Neither the shore power pedestal breakers nor the Neutral conductor are overloaded.  All is safe and well within specifications.

At the end of the evening, when our sample boaters retire to bed, assume they turn off all of the house loads.  The hot water heater is satisfied, the battery charger is satisfied, the fridge is satisfied, the TV is “off,” the laptop and iGadget batteries are charged (and the screens have gone “dark”), and the space lighting is “off.”  Now, there is no current at all flowing in the House loads.  Ah, yes, but the air conditioning is still needed.

Figure 7 represents the electrical status in this case.  Since the heat pumps are still running, there are 20A flowing in the heat pump circuit.  Since there is nothing “on” in the House load group, the arithmetic difference is 20 amps, which returns on the neutral (N) conductor.  Again, no part of the circuit carries more than a total of 20A.





Use Case 3: a boat wired with a 125V/250V, 50A shore power cord, but fit with 240V appliance loads aboard.

Figure 8 shows the addition of pure 240V loads at the far right of the drawing.  Boats with 125V/250V, 50A shore power service which have both 120V and 240V appliance loads (hot water heater, cooktop, electric dryer, heat pump compressor) are electrically very similar to those without 240V appliances.  Very few “240V appliances” are “pure” 240V devices.  The only ones that come to mind are 2-pole, 240V deep well pumps and 2-pole, 240V hot water heaters.  Appliances like heat pumps, cook tops, ovens, clothes dryers and watermakers, are usually “hybrid devices;” ie, they need both 120V and 240V to operate.  The control circuits in hybrid appliances are generally 120V circuits.  In a dryer, for example, the heating elements are 240V but the motor that turns the drum and the clock timer circuit both require 120V.  Hot water heaters can be pure 240V-only loads which do not need or have a neutral conductor.

In Figure 8, the pure 240V appliance loads are electrically in parallel with the two 120V series loads, and the 240V loads add to the amps drawn in the 120V supply mains, L1 and L2.  So, if we had the 20A L2 load running a 120V heat pump, as has been the example throughout this article, and in addition, a 240V hot water heater simultaneously calling for 12A, the result would be a 32A total Amps in L2.  Attached to a 50A pedestal, all would be OK, but attached to a 30A splitter, the result would be a tripped 30A pedestal circuit breaker.  So again for emphasis, it is up to the boat owner/operator to understand load management and ensure that pedestal breaker capacity is not exceeded.

Potential Power Issues with Certain Shore Power Transformers

The utility power on docks can originate from two kinds of public utility sources.  “Single phase” sources will appear as conventional 120V/240V.  “Three phase” sources will appear as 120V/208V.  Because this electrical fact is a well-understood, and very common in boating, UL Marine certified electrical appliances are designed to accommodate the difference between 240V and 208V.  Residential appliances MAY NOT have have that same flexibility.

Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for  125V/250V, 50A applications   are manufactured in three “flavors:”

  1. Basic, single input, single output, 240V transformer; least expensive flavor.
  2. Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input to achieve a constant 240V output.
  3. High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage.   While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.

Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction.   Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.”   The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor.    The output of this transformer (the secondary) provides single phase, 3-pole, 4-wire power to the boat. In English, that means there is a conventional black, red, white and green output.    If the input voltage to a basic style transformer is 240V, the output will be 120V/240V.   But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances.   With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.

One hundred four volts is a low utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances.  Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally.  Microwaves will run but will take slightly longer to cook.   Coffee pots will perk, but will take slightly longer to perk.    Electric blankets will keep sleepers warm and cozy.   Water Heaters will heat water, but take slightly longer to reach target temperature.   Stovetop burners will heat, but not get as hot at the same setting.  Heat pump compressors and fans should all run, but some motors may overheat and cut out to protect themselves from damage.  Marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible.   One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V should be designed to detect the condition, put up a power warning fault light, and self-disconnect.    Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer marine heat pump designs do that.

WARNING:  if there is a 240V shore power supply voltage applied to a manual transformer set to a 208V input voltage, then the AC voltages aboard can get high enough to “damage” appliances.

Article Summary:

  1. When operating a 125V/250V, 50A boat which does not have 240V loads, total loads of up to 2 * 3600 Watts can be supported with two conventional 30A pedestal outlets.  In this case, neither the energized (hot) conductors nor the Neutral conductor are ever overloaded.  No individual circuit conductor ever conducts more than 30A.
  2. When operating a boat with pure 240V loads, the Amps required by the 240V loads add to the Amps needed in the 120V loads.  The owner/operator must monitor total amps drawn/power used to keep total power consumed below 3600 Watts per side.
  3. Some shore power circuit breakers are housed in inaccessible, locked locations ashore.  If a boater accidentally trips a shore power circuit breaker, particularly after hours, it may not be possible to gain access to it in order to reset it
  4. It is necessary for boat owner’s to closely monitor power usage and limit the amount of  current used to prevent tripping shore power circuit breakers.  Care must be exercised to not run high amp draw appliances (coffee pots, microwave ovens, inductive cookware, hair dryers, clothes washer/dryers and similar devices) at the same time.  Boats with multiple heat pumps will probably be unable to run all of them at the same time on 30A services.
  5. The examples in this article assume that the heat pump circuit is on one 30A load leg and house loads are on the other leg.  Obviously, some boats are wired differently. Systems with heat pumps and house loads distributed across both incoming energized 120V legs will have to monitor loads and current draws in the same manner, but the electrical principles discussed above remain the same.
  6. The specific balance of currents in the load one group and the load two group changes constantly.  L1, L2 and Neutral current (Amps) never exceeds 30A.

Appendix 1:

To the right is the electrical diagram of a typical “Smart Wye” splitter.  This Figure represents the electrical circuit detail of the splitter shown in Figure 2 in the earlier text.  Note that the splitter contains a relay – labeled “K” in the drawing.  The relay requires 208V or 240V to close.  Without at least 208V, the relay will not close and the splitter will not pass any power through to the boat.

Following is a link to my article describing Smart Splitters, and the receptacles required for their successful operation.

AC Electricity Fundamentals – Part 2: The Boat AC Electric System

Article posted: April 20, 2019
Added content: Shore Power Transformers; July 22, 2019

About this article

The AC Electricity Fundamentals – Part 1 article precedes this article and discusses 1) the concepts, terminology, components and layout of National Power Grid generating equipment, 2) delivery of AC power into residential neighborhoods, and 3) the configuration of AC electrical systems within a residential building, all at an introductory level. An understanding of residential AC power systems is foundational to an understanding of AC power systems on boats.

The Part 1 article concluded by showing that the AC shore power system on boats is equivalent to a sub-panel in a terrestrial building. In the NEC architecture of terrestrial AC building systems, sub-panels are subordinate to the main service entrance panel of the building. In the same way, boats are subordinate to the shore power AC electrical infrastructure of a terrestrial facility.

This article focuses on the overall AC electric “platform” aboard cruising boats. On boats, shore power is only one component of a typical AC system “platform,” which can also include a mix of onboard generator(s), inverter(s) and in some cases, shore power transformer(s). This introductory Part 2 article will answer some questions, and will undoubtedly raise others. The goal of this article is to help readers understand AC electrical concepts and topics to be able to discuss questions, concerns, symptoms and options with marine-certified, professional electrical technicians.

Personal Safety

Virtually all electricity can be dangerous to property and life. Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard. This is especially true of inverter-chargers. The large batteries found on boats can produce explosive gasses and store enough energy to start a large, damaging fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity! Anyone working in noisy environments, with running engines or other loud machinery, MUST WEAR HEARING PROTECTION.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right tools for a job…
If you are not sure you know how to use the tools you do have…
Well, then, LEAVE IT ALONE until you learn more!


Electrocution is a biological insult arising from an electric shock that paralyzes either the respiratory or cardiac functions of the body, or both. Electrocution results in death. Even very small electric currents, under the right circumstances, can result in electrocution. Obviously, electric shock can be a life threatening emergency.

If you are present and witness an electric shock or electrocution, in any locale around boats or water, there are several things that need to be done immediately. Remember, since the victim is not breathing, you’ll have 5 minutes or less to accomplish items 3 – 10, below:

  1. STAY CALM! You can not save someone else if you panic!
  3. SCREAM FOR HELP! ATTRACT ATTENTION! Point at the first person who’s attention you get and instruct them to “call 911 for an electrocution!”
  5. If the victim is in the water, KILL POWER TO THE ENTIRE DOCK.
  7. After power is removed, raise the face of an unconscious victim out of the water.
  8. After power is removed and the victim’s airway is secured above water, if help has not arrived, call 911 again! Two 911 calls are better than none.
  9. After power is removed, and with access to the victim, assess victim and initiate CPR as appropriate. CPR is often successful in reviving or saving electrocution victims who are otherwise healthy at the time of the accident.

Boat Electrical System – Scope

Viewing the shore power AC system of a boat as a residential sub-panel in a single family residence is simple and technically accurate, but the AC electrical system of a typical cruising boat is more than just shore power. A simple block diagram of a boat electrical platform can show the relationships among the various components of the boat’s AC (and DC) electrical systems. Sanctuary’s energy flow diagram is shown in Figure 1. This diagram shows Sanctuary as she is today. When we bought her, she did not have a genset, she did not have an inverter-charger, and she was fit with two inappropriate battery banks. She was less complex yet poorly designed for our intended use.

Figure 1 shows Sanctuary’s AC and DC electrical systems as a complete and integrated operational “platform.” From the platform perspective, owners can evaluate the impacts of contemplated alterations and upgrades. The diagram shows energy flow, not wiring detail.   Its simplicity allows one to visualize and understand both individual components and how components feed and are fed by one another. It is all too easy to add, remove and change system components without fully appreciating the impact(s) to the overall host electrical platform.

Sanctuary’s OEM factory configuration consisted of two 8D batteries, one dedicated to engine starting, and the other dedicated to modest OEM space and navigation lighting loads. An 8D was excessive and poorly utilized for engine starting, and inadequate for our house needs. The energy flow diagram gave us the ability to visualize the impacts of consolidating the two separate battery banks into a single bank.

Sanctuary’s OEM factory battery charger was an obsolete technology single-stage unit. We wanted AC power aboard without having to run our genset. We decided to change the battery charger to a fully automatic inverter-charger. This was a major upgrade that affected both the AC and DC electrical systems aboard. Mechanically, the upgrade was simple, but modification of branch circuit wiring to comply with the ABYC electrical standard was a big impact to our host AC electrical system.

Adding a new genset to a boat includes adding a Generator Transfer Switch and reworking the distribution wiring of the existing shore power circuits. Replacing an old-iron 60Hz AC genset with a new 60Hz AC genset would be relatively easy and non-disruptive. Converting to a DC genset (a diesel-driven DC battery charger) has very different implications. Cost, technical complexity and value of the alternatives can be compared and evaluated.

The energy flow diagram shows that Sanctuary is now fit with a single battery bank for both “engine start” and “house” support. Adding a wind generator or adding solar panels are energy management solutions that would have technical impacts to the existing system. Each can be evaluated from the perspective of our energy flow diagram. Boat owners are strongly encouraged to take a “platform view” as opposed to a “component view” of the electrical systems on their boats.

High Complexity Aboard Boats – Power Sources

What is immediately clear from Sanctuary’s energy flow diagram is that there are three entirely independent AC sources that can feed power to our onboard AC loads:
1. shore power (source ashore),
2. generator (source aboard),
3. inverter, (source aboard) and on some boats,
4. shore power transformers (source aboard)(not installed aboard Sanctuary.

Key Electrical Concepts For Boats

Key points from the Part 1 article to keep in mind on boats:

  1. “Shore power” arises from the electrical system of a terrestrial facility, ashore, while AC power from a “generator,” “inverter,” and/or “shore power transformer” arises from equipment installed aboard the boat.
  2. The residential AC power standard in North America is a “Single Phase, Center Tapped, Three-Pole” grounded-neutral system. This definition broadly applies to all terrestrial buildings with which people interact, and includes boats.
  3. State/Province, county and municipal jurisdictions across North America adopt local statutes and codes-of-regulations that originate with the NEC/CSA to govern terrestrial building electrical installations.
  4. There are no statutory electrical codes for boats. The American Boat and Yacht Council (ABYC) provides voluntary standards to boat builders. ABYC electrical standards are fully compatible with NEC shore power, assuring safe, reliable inter-operability between terrestrial and boat-resident AC systems.

As discussed in the Part 1 article, an essential safety requirement of all of these standards and codes is that single phase electrical systems be “grounded” at their “derived source.” This brings us face-to-face with some core ABYC “recommendations” that govern switching of AC wiring for equipment installations on boats:

  1. only one source is allowed to power loads aboard boats at any one time,
  2. sources must be thoroughly and completely isolated from one another,
  3. a “grounded neutral system” is required:
    • when on shore power, the neutral-to-ground connection is provided to the boat through the shore power cord, (i.e., the neutral-to-ground connection is in the shore power infrastructure), and
    • when on generator or inverter power, or when shore power is received through an onboard shore power transformer, the neutral-to-ground connection is made at the onboard source.

High Complexity Aboard Boats – Ground

In a “Single Phase, Center Tapped, Three-Pole” grounded-neutral system, what does “grounded neutral” mean? Recall in the residential AC system model that three conductors arise from the utility power transformer at the street; two energized lines (“L1” and “L2”) and one neutral line (“N”). As these three lines emerge from the utility transformer in the street, 240V are present between “L1” and “L2,” and 120V between “L1” and “N” and between “L2” and “N,” but these voltages “float” with respect to their external environmental surroundings (recall the discussion of birds and squirrels on wires from Part 1). This situation is referred to as a “floating neutral.” To create a safe, known zero-volt system reference, copper rods are driven into the earth at the building’s service entrance location. Within the main service panel of the building, the utility-provided neutral conductor is connected (“bonded”) to this network of copper ground rods. The result is an earth-ground “grounded neutral” system.

Grounding the neutral is very straight-forward at buildings. Since there is only one place where utility power enters the building from the utility company’s electric meter, it’s easy to understand and visualize that entrance location as the “derived source” of the power. Electrician’s working in terrestrial buildings learn to mix neutral and safety ground conductors on the same buss bars in the main service panel. In one of the examples I showed in the Part 1 article, we saw that some main service panels are built with only one buss bar which serves to collect both neutrals and grounds.

Boats are different!  In the architecture of the North American power framework, boats are sub-panels to the shore power infrastructure, not main service panels. Furthermore, it is common to have more than one AC power source for the AC system platform on a boat, including as we saw in Figure 2, AC Shore Power connections, onboard generators, inverters or inverter-chargers, and maybe shore power transformers (isolation transformer, polarization transformer). All of these sources are AC “derived sources” within the definitions of the ABYC Electrical Standard, E-11.

The NEC requires the neutral-to-ground bond to be at the “newly derived source” of the terrestrial shore power system. The ABYC electrical standard complements and supports the NEC requirement for boats operating on shore power. For boats operating on shore power, neutral-to-ground connections are not permitted aboard the boat. Why? Follow this scenario:

  1. Start with the NEC-required neutral-to-ground bond being correctly installed at the terrestrial facility’s main service panel (derived source) .
  2. The shore power neutral-to-ground bond is carried aboard the boat via the shore power cord, per ABYC E-11,
  3. The intent of the safety ground is to provide a low resistance electrical path to disconnect power as close as possible to the source in an electrical emergency:
    • in a normal AC system, no power flows in the safety ground conductor,
    • but in the case of an electrical fault, current flows in the safety ground for the purpose of removing power (fault removal) by tripping the circuit breaker that feeds the faulting circuit),
  4. Because there is a neutral-to-ground bond in the shore power main service panel, if there were also a neutral-to-ground bond aboard the boat, the neutral and ground conductors between the shore infrastructure and the boat would be electrically in parallel with each other, enabling power to flow in the safety ground (by definition, a ground fault). This results in two issues for boaters:
    • constantly trips a dockside ground fault sensing circuit breaker, and
    • the AC safety ground would, itself, be energized, thus providing a path to the underwater metals of the boat, thus enabling AC power to escape the boat’s electrical system into the water.
  5. The above consequences of paralleling the neutral and the safety ground pose a shock and electrocution threat to people, pets and wildlife in the water.

So, now we understand why a neutral-to-ground bond is not permitted aboard the boat when connected to shore power. But, we also know that ABYC does require a neutral-to-ground bond for onboard generators, inverters operating in “invert” mode and shore power transformers; that is, ABYC requires a grounded-neutral AC system throughout the boat regardless of the source of AC power.

Summarizing the above:  Shore power can’t have a neutral-to-ground bond aboard the boat, but generators and inverters must have neutral-to-ground bonds at the respective equipment aboard the boat. Isn’t this an irreconcilable “Catch-22?” In a word, “no!” It is a complex wiring situation that does not occur in terrestrial buildings where only one power source is present. (It does apply in terrestrial buildings if an outdoor emergency generator installed, and it also occurs in terrestrial off-grid solar applications.)

The technical solution that allows compliance with these apparently self-contradictory ABYC configuration requirements involves complex switching solutions. When connected to shore power, onboard neutral-to-ground bond connections must be “switched out.” When running on an onboard generator or an inverter in “invert” mode, the neutral-to-ground bond connection must be “switched in.”

High Complexity Aboard Boats – Switching

Marine-certified AC disconnect circuit breakers are readily available in a variety of form factors to fit different power panels of different companies found on different boats. With 120VAC, 30A inlet circuits, “Double Pole” breakers disconnect the “L1” and “N” lines. With 240VAC, 50A inlet circuits, “Double Pole” breakers disconnect “L1 and “L2,” but not “N”. It is up to the installing electrical technician to ensure that the correct disconnect breakers are used in the correct application to maintain compliance with the ABYC electrical standard and compatibility with the shore power infrastructure.

Looking at Sanctuary’s energy flow diagram, Figure 1, we can see that the boat’s Generator Transfer Switch (GTS) is used to transfer the “load” (the “load” in this case is the boat’s entire AC electrical system) between one of two AC power sources (either shore power or the onboard generator). The GTS must be constructed in a way that it simultaneously transfers the load’s “hot” lines (“L1 and L2”) and the load’s “neutrals” “N.” Figure 3 shows the electrical diagram of Sanctuary’s physical GTS. “Source 1” and “Source 2” are our 120V, 30A shore power inlets. “Source 3” is our 240V, 50A generator input (happens to be the way our generator is configured). In order to comply with the neutral-to-ground bonding requirements of the NEC and ABYC, the GTS is built to switch the neutrals as well as the “hot” lines. In this way, the required neutral-to-ground bond can be installed at the generator, aboard the boat, and the entire platform remains compliant with the ABYC electrical standard and compatible with the NEC for shore power.

About Shore Power Transformers

Shore power transformers are expensive, large, heavy and require significant physical space surrounded by free-flowing air for ventilation. These transformers can suppress spikes and electrical noise from entering the boat from the shore power grid. Some transformer designs can automatically compensate for “low” dock voltage (shore power “brownout,” normal 208VAC). There are two shore power transformer wiring configurations: an “isolation configuration” and a “polarization configuration.” In both cases, the transformer is installed aboard the boat. The secondary winding of the shore power transformer is defined to be the “derived source” of AC power.

For a 30A, 120V isolation transformer, the primary requires a double pole breaker, preferably fit with ELCI, which breaks both “L1” and the neutral, “N.” The safety ground in the shore power cord is connected to an internal shield inside the transformer but does not continue to the external case of the transformer. The boat’s safety ground originates at the transformer’s external metal case. The transformer is the derived source, so the neutral and the safety ground are bonded together at the transformer. The boat’s physical safety ground network does not connect back to the shore power infrastructure. The secondary winding feeds onboard 120V branch circuits.

For a 50A, 240V isolation transformer, the “L1” and “L2” hot lines are brought aboard through a double pole disconnect breaker, preferably fit with ELCI. The pedestal neutral, N, is not brought aboard at all. The safety ground in the shore power cord is connected to an internal shield inside the transformer but does not continue to the external case of the transformer. The boat’s safety ground originates at the transformer’s external metal case. The transformer is the derived source, so the neutral and the safety ground are bonded together at the transformer. The boat’s physical safety ground network does not connect back to the shore power infrastructure. The secondary winding feeds onboard 120V/240V branch circuits.

The difference between “isolation” and “polarization” is the wiring configuration of the safety ground. With isolation transformers, the safety ground in the shore power cord terminates at a shield in the transformer. With polarization transformers, the safety ground of the shore power cord is connected to the boat’s safety ground buss, and is brought back to the shore power pedestal. With a polarization transformer, it is best practice to also install a Galvanic Isolator in the safety ground wire.

Shore Power transformers are available for both 125V-only and 125V/250V applications. Shore Power transformers for 125V/250V, 50A and 125V/250V, 100A applications  are manufactured in three “flavors:”

1. Basic, single input, single output, 240V transformer; least expensive flavor.
2. Multiple, selectable input-voltage taps; manual switching allows the user to select back-and-forth between 208V input and 240V input for 240V output.
3. High-end transformers; sense the input voltage to automatically maintain the desired 240V output voltage.  While this is the best choice for most boaters, it is also the most expensive, so is not usually found on spec-built boats.

Owners of boats fit with shore power transformers must be especially aware of their transformer’s construction.  Basic 125V/250V, 50A, single input, single output transformers are wound with a ratio of primary windings to secondary windings of one-to-one; written this way: “1:1.”  The input of this transformer (the primary) is a two-pole connection where there is no Neutral conductor.   The output of this transformer (the secondary) produces single phase, 3-pole, 4-wire output which powers the boat.   In English, that means there is a conventional black, red, white and green output.   If the input voltage to a basic style transformer is 240V, the output will be 120V/240V.  But, if the input voltage to a basic style transformer is 208V, the output will be 104V/208V, which may be problematic with some 120V AC appliances. With a 1:1 winding ratio, the leg-to-leg output voltage (secondary) would be 208V instead of 240V, and the leg-to-neutral voltage would be only 104V, instead of 120V.

One hundred four volts is a low utility outlet voltage, and although alarming to most users, it is NOT “too low” for most modern AC home appliances. Modern TVs, DVRs, computers, SOHO wi-fi routers and entertainment systems should all run normally.   Microwaves will run but will take longer to cook.  Coffee pots will perk, but will take longer to do their thing.   Electric blankets will keep sleepers warm and cozy.  Water Heaters will heat water, but take longer to reach target temperature.   Stovetop burners will heat, but not get as hot. Heat pump compressors and fans should all run, but some motors may overheat and cut out to protect themselves from damage; marine refrigerators have 12V DC compressors (or 24V DC compressors), and are unaffected by AC supply voltages, but household appliances (refrigerators, freezers, ice makers) used on boats may not be as flexible.  One hundred four volts is the low end of the “brownout tolerance” for AC appliances. Any marine appliance that would be damaged by, or fail to perform properly at, 104V “low voltage” should be designed to detect the condition, put up a power warning fault light, and self-disconnect.   Many mobile (marine, RV, emergency vehicle) inverters and inverter/chargers and newer 120V marine heat pumps do that.

About Generators

An AC generator is a mechanical machine consisting of a propulsion engine that drives an alternator. The machine must spin at a constant rotational speed to maintain the 60Hz output frequency. The waveform from a rotating genset is a pure sine wave. Although gensets are rarely actually run at their full load capacity, AC gensets must be rated for the largest electrical load they will ever have to support. Mechanical speed controls in these machines add to the requirement for a relatively great deal of preventive and corrective maintenance. Replacement parts are expensive and heavy. An AC generator can power all normal household appliances including heat pumps. Considering capital expense and lifetime fuel and maintenance costs, AC gensets are inherently expensive, per kW-h, to produce AC electricity on a boat.

A DC generator can be a practical alternative to an AC genset for most cruising boats.    DC gensets such as those made by Alten®, Hamilton-Ferris®, PolarPower® and ZRD® are essentially used aboard as “motor-driven battery chargers.”  The AC power used aboard the boat arises from the battery bank via inverter(s).  Multiple smaller inverters can provide for staged comfort and convenience options as well as systems redundancy.   Because batteries can supplement total power demand (Kirchhoff’s Laws), DC gensets do not have to be rated for max demand, as do AC gensets.  When onboard loads are light, the DC genset provides enough energy to power both the inverter(s) (for conversion to AC) and the battery bank (for battery charging).  When demand exceeds the generator output capacity, the batteries themselves make up the difference.  This means DC gensets can be of smaller capacity and can adjust to light loads more efficiently than AC gensets. Since DC gensets charge batteries, they do not need to spin at a regulated speed and are mechanically less complex.  AC generators are sensitive to rotational speed to keep the AC output frequency at 60Hz, +/- two Hz.  The DC machine has no such restriction, and so are much more fuel efficient. Boaters faced with installing a net new genset or replacing an old genset would do well to consider the DC genset option.

High Complexity Aboard Boats – Inverter

From the perspective of “electrical standards,” boats are a sub-class of a larger category of “mobile platforms.” Inverter and inverter-charger devices can be installed in many types of mobile platforms, including cars, trucks, ambulances, emergency vehicles, RVs and airplanes. All classes of “mobile platform” have identical shore power interface compatibility requirements, and very similar user safety requirements. Inverters installed in host AC systems on boats carry significant complexity.

About Inverter-Chargers

An “inverter-charger” is an electronic device that converts DC from batteries into 120V/240V, 50Hz/60Hz AC and ALSO uses 120V/240VAC, when available from external sources, to re-charge battery banks. The shape of the AC waveform from inverters can be a “modified sine wave” (MSW) or a “pure sine wave” (PSW). PSW devices dominate in the marketplace in 2019, and since some electronic appliances do not tolerate MSW well, are to be preferred aboard boats.

There are two installation use cases that apply to any discussion of inverter or inverter-charger installations on boats.

Use case one:  consists of a stand-alone inverter that powers dedicated AC utility outlets that are separate and apart from the wiring and outlets of the host boat’s main AC electrical system. To have AC power at those outlets, the inverter must be turned “on.” When the Inverter is turned “off,” AC power is “off.” The AC wiring attached to this inverter would be expected to comply to the normal requirements for all AC wiring aboard. There is no automatic power transfer switching. Ideally, an inverter used in this way would feed a distribution panel that would provide overload protection to branch circuit wiring. The manual nature of this use case is not considered “desirable” by boat designers and builders. Specific standards for this use case are not enumerated in the ABYC E-11 standard, AC and DC Electrical Systems on Boats.

Use case two: an inverter or inverter/charger that is fully integrated into, and functions as a part of, the host boat’s AC electrical system.   There are no separate or isolated utility outlets. All powered utility outlets are overload-protected by the host system’s branch circuit distribution panel. Branch circuit utility outlets and appliances either 1) receive externally-provided AC power “passed through” the inverter or 2) receive AC power from the inverter via the energy stored in the boat’s batteries.   The inverter senses loss of external power automatically, and switching from external power to battery power is likewise automatic. User safety and convenience is maximized. This use case is covered in detail by ABYC E-11, AC and DC Electrical Systems on Boats, and ABYC A-31, Battery Chargers and Inverters. ABYC specifies device compliance with UL458 to maintain compatibility with neutral-to-ground switching aboard the boat.
As shown in Figure 4, when either shore power or generator power is available, the inverter automatically switches to “standby mode.” In “standby mode,” the internal transfer relay is energized by the external power source. The internal transfer relay has two functions. One is to pass the external power through the inverter (“passthru”) to the boat’s power distribution panel (red arrow), and the other is to simultaneously remove the device’s internal neutral-to-ground bond (red oval). This second function maintains compliance with the ABYC neutral-to-ground bonding requirements for shore power.Later, when external power is no longer present, the device automatically switches from “standby mode” to “invert mode.” As shown in Figure 5, the internal transfer relay drops, and the inverter begins to generate AC power by drawing energy from the boat’s batteries (red arrow). When the transfer relay drops, it simultaneously establishes the required neutral-to-ground bond (red oval). Since the inverter in “invert mode” is now the “derived source” of AC power, grounding the neutral via the internal relay maintains compliance with the ABYC electrical standard, E-11.

Inverters – Installation Impacts

Referring again to Figure 1, the energy flow diagram for Sanctuary, it is apparent that the 120V feed of branch circuits 1 – 3 and 4 are powered from either shore power or generator power through the Generator Transfer Switch. When on shore power or generator power is present, the inverter operates in “standby mode,” and AC for branch circuits 5 – 8 “passes through” the power transfer relay of the inverter-charger to feed AC to those circuits. When the boat is under way, and external power is not present, the inverter switches to “invert mode.” In that case, branch circuits 5 – 8 are powered by the inverter-charger.

What is not obvious in the energy flow diagram is that, because the “hot” lines for circuits 5 – 8 originate at the inverter, the neutrals for circuits 5-8 must be separated from the neutrals of circuits 1-4. This is a manufacturer’s installation requirement for the inverter-charger device which has its origins in ABYC Standard, A-31, Battery Chargers and Inverters.

Inverters – Advanced Feature(s)

In 2019 in worldwide boating markets, Victron Energy B.V.® manufactures a series of inverter-chargers carrying the MultiPlus™ and Quattro™ brand names that have an advanced feature Victron® calls “Power Assist.” With this feature, the inverter is capable of “piggybacking” on top of a limited shore power source to boost the total amount of power available to power loads aboard the boat. Batteries are charged during periods of low demand, and support the inverter during periods of higher demand. Across a day of use, users must monitor the system to assure batteries are adequately charged.

A typical “Power Assist” scenario: assume a boat fit with one of these inverters visits a private residential dock, a public wall, or any similar location where only very limited AC shore power is available from a single 125V, 15A/20A residential outlet. Generally, 15A is not sufficient for powering boat loads by itself. That said, if the demand on the 15A circuit can be held below a level that causes the shore power overload circuit breaker to trip, convenience aboard the boat can be enhanced by the “Power Assist” feature. To ensure the inverter does not trip the shore power circuit breaker, assume the inverter’s shore power “Maximum Current” setting is 12A. As long as the “passthru” loads on the boat are less than 12A, the power for those loads comes entirely from the shore power outlet.  It is during these periods of light AC loads aboard the boat that house batteries are charged.

Later there may come a time that the load on the boat jumps up, perhaps because of a toaster, coffee pot, microwave or hair dryer. Assume that at some point the total AC load aboard the boat rises to – pick a number – 22A. Since the inverter-charger is limited to drawing 12A from the shore power outlet, the inverter itself jumps in to “assist” the shore power source with energy drawn from the boat’s batteries. The inverter will sync with the shore power sine wave, and 10A will be provided from the batteries by the inverter. Keep in mind that the inverter is designed to provide this assistance automatically, by monitoring passthru load and automatically jumping in to supplement loads that exceed the pre-set.

Functionally, the above is how the Victron® Power Assist feature works, and it has much user convenience appeal to boaters. However, there may also be an operational downside with the “Power Assist” feature. When this equipment attaches to the Electric Power Grid, it synchronizes it’s 60Hz power waveform with the power on the grid. Emerging experience suggests the synchronization process can cause out-of-phase currents that may trip dockside ground fault sensors.  Owners of these devices should be alert to nuisance trips when connecting to docks with ground fault sensors on pedestals.

Inverters without the “Power Assist” feature have an obvious “one-way” relationship with the Electric Power Grid; that is, they are loads that take power from the grid. Inverters with the “Power Assist” feature are electrically paralleled to the incoming shore power connection and can have a two-way interface with the incoming AC power grid. These two-way inverters are capable of delivering AC power backwards into the electric power grid to which they are attached. The ability to feed power backwards into the grid carries significant safety implications in certain fault scenarios.

“Distributed Energy Resources” (DERs) are AC electricity generating units, typically in the range of 3 kW to 50 mW, that are deployed across the power grid. DERs are installed close to loads, often on customer premises, often on the load side of the customer’s electric meter. DERs are designed to alternately draw power from and return power to the upstream hosting electrical power grid. Worldwide, DERs are a central concept to distributed solar and wind farm (“green energy”) production and to pumped-storage reservoir systems. DER technologies include 25kW to 500kW micro-turbines, 25kW to 250mW combustion turbines, 5kW to 7mW internal combustion engines, 1kW to 25kW Stirling engines, fuel cells, battery-based UPS systems, photovoltaic systems, and wind generation systems.

In the US, the NEC, state Public Utility Commissions, code enforcement Authorities Having Jurisdiction (AHJ), and the ABYC, have all recognized the safety implications related to DERs. While it would be rare – in 2019 – for power generated on a boat to be fed back into the local electric power grid, with a DER-capable inverter, it is possible. The “Power Assist” capability enhances living convenience for boaters as it does for land-based DER users, so its likely that inverter-type DER devices for applications aboard boats will only increase in availability in the future.

ABYC A-32, July 2017, is the most current electrical standard that governs the two-way interface of DER equipment when installed on boats. ABYC, A-32, AC Power Conversion Equipment and Systems, Diagram 1, is shown below. This diagram is the electrical “model” the ABYC has adopted for inverter-type DERs installed on boats. Referring to this diagram, the earlier discussion of neutral-to-ground bonding still applies. The relay that accomplishes that is shown in the green oval.

Inverter Safety – “Anti-Islanding”

In residential neighborhoods (and aboard boats), power arises from the local Electric Power Utility. If power is lost, the implication is that some part of the utility power grid failed. Causes can include electrical device failure, severe weather, floods, terrorism or severe mechanical insult (tree-fall on wires, vehicle into utility pole, hot air balloon into wires, etc). A loss-of-power event leaves some local geography without electricity; home(s), police/fire station, shopping center, hospital, farm, airport, etc., an entire neighborhood, a entire town, etc. Many affected entities have mission-critical needs for uninterrupted power, and use DERs to achieve that goal. The footprint of the area of lost power is referred to as an “Island;” that is, an area that is physically cut-off and isolated from the power grid.

For the safety of residents, rescue personnel and repair personnel working to restore power within the “island” of disruption, DER’s operating at the time of a power failure must immediately detect the loss of grid power and disconnect themselves to prevent back-feeding power into the “island.” Again referring to the ABYC diagram, the relay shown in the red oval is the means by which DER Inverters disconnect themselves from the grid. ABYC requires that the disconnect occur within 100mS of the loss of power. Note: the inverter may continue powering some or all of its attached loads, within the rated capacity of the inverter and the capability of the battery bank.

Boaters are NOT expected to understand or care how all this happens.  The net here is, boaters need to buy and install MARINE-CERTIFIED equipment for installation aboard their boats. Equipment from discounters like Harbor Freight does not meet these complex safety requirements.

Behind the words “MARINE-CERTIFIED” is a very complex series of electrical standards that spans the worldwide membership of the IEC. These standards define the mutually-cooperative manner in which DERs must interact with National Electric Power Grids. At the end of this article is “Addendum 1” that describes the safety and testing standards involved with DER equipment for those interested.

About Motors – Single-Phase

Single phase motors are more complicated than three-phase motors. Even small sized single-phase motors are more complicated – electrically and mechanically – than three-phase motors. The reason is that it is much more difficult to create a rotating magnetic field with just one, single-phase. The “natural” rotation of the phases of a three-phase machine does not exist in a single-phase machine.

There are several different techniques used to create a rotating magnetic field in a single phase motor. All of these motors have high inrush “surge” currents.

A shaded-pole induction motor is a relatively simple and inexpensive motor. There are no brushes. Starting torque is low, so these motors are used for fan and blower motors and other low-starting torque applications. Creation of the torque to start rotation is done by means of one or two turns of heavy copper wire around one corner of the field coil. When the field is energized, inrush current is induced in this heavy coil. This induced current is out-of-phase with the power line current. This results in a second, offset, magnetic field, which is enough to start motor rotation. These motors are generally made in fractional-horsepower sizes.

Where medium and medium-high starting torques are required, the split-phase induction motor is more appropriate. These motors also do not have brushes. Split-phase induction motors are built with two field windings. One of the windings is called the “start” winding and the other is called the “run” winding. One of the windings is fed with an out-of-phase current to create a rotating magnetic field. The out-of-phase current is commonly created by feeding the winding through a capacitor. A common variation of this design is a switch that disconnects the capacitor when the motor is up to operational speed. In this design, a centrifugal switch is internally mounted to the armature. The switch opens to disconnect the start capacitor when the rotor reaches operating speeds. Often in motors of this type, there is an audible click of the centrifugal switch transfer as it opens and closes. This is normal. In compressor applications, another variation is to have capacitors in both the start coil circuit and the run coil circuit. These alternatives involve complexity and cost.

In addition to a start/run capacitor, another way to achieve a rotating magnetic field is with a second field winding with significantly different values of inductance from the main winding.  This effectively results in an out-of-phase current in the second winding.

Where small physical size and high torques are needed, the Universal Motor is preferred. Universal motors are expensive to build and require periodic maintenance. These motors have carbon brushes and complex internal components that create a strong, consistent magnetic field at all rotational speeds. They can start to rotate against high stall loads. These are commonly used in handheld tools (drills, saws, etc.) and kitchen appliances like mixers and blenders. These motors often are not rated for continuous use, because they generate significant internal heat in operation.

About Motors – Three-Phase

Three-phase motors are very simple electrical machines. Recall that in a generator, there was a rotating magnetic field inside three fixed armature spaced at intervals of 120°. Three-phase motors have field coils that are physically mounted at intervals of 120°. The incoming three-phase power is connected to the windings of the motor’s field coils. As the voltage in the phases rises and falls, each in turn, in the 60Hz sinusoidal rhythm, a magnetic field strengthens and weakens around the field coils. An aggregate rotating magnetic field is produced by the rise and fall of current in the three individual field coils. That aggregate magnetic field rotates around the diameter of the machine’s field coils at a rate of 60 times per second. Reversing the connections of any two of the incoming three phases will reverse the direction of rotation of the magnetic field, and therefore, the direction of rotation of the motor itself.

A characteristic of motors is that they have high start-surge currents. At the moment when power is first applied to the machine, this surge is at its greatest. As the motor spins up to its running speed, the current settles down to its steady-state running level. Motors have separate ratings for start and run currents. Circuit designers need to allow for start-surge currents in selecting the gauge of wiring to the motor. Large horsepower motors have special controllers that limit inrush surge, but small frame motors found in boats generally do not need these sophisticated controllers. Because of the inrush surge, motor circuits are generally set up with slow-blow circuit protection.

The strength of the magnetic field determines the amount of torque the motor can deliver. The work will be to turn pumps, fans, windshield wipers, machine tools, refrigeration compressors, etc. Starting torque is large because of large start-surge currents. Running torque is the steady state torque the motor produces. Engineers select motors to match the torque required by the machinery the motor will drive.

About Motors – Raw Water Pumps

In motor-driven water pumps used in terrestrial applications – a residential hydronic heating systems, for example – an electric motor connects to the pump via a mechanical shaft. A rubber “lip seal” is used in the pump housing to prevent leaks at the shaft. This design has it’s limitations. Over time, the lip seal will harden, crack and fail and/or the shaft will become scored from mechanical wear, leading to leaks. Obviously, this design represents a future maintenance activity for the owner.

Boat raw water pumps are of different design. Instead of a mechanical shaft, the motor is fit to a strong permanent magnet. The pump impeller is also magnetic, and rotates on a shaft mounted inside a Fiberglass Reinforced Plastic (FRP) housing. The pump housing is designed so that when fit together with the motor, the magnet fits inside the metal-containing impeller’s housing. Since there is no shaft penetration through the housing, there is nothing to leak. As the motor spins, the magnetic field acts through the FRP housing and causes the impeller to spin. Good installation practice is for the assembled motor and pump be mounted vertically with the motor above the pump.

This design is leak free. The impeller can jam, but the pump motor will not overheat and will not be damaged if it does. These motors generally need little maintenance, but check the manufacturers instructions to verify the needs of your pump motor.

About Motors – Maintenance

Routine maintenance for electric motors includes, first and foremost, periodic lubrication of sleeve bearings. Use machine oil, not automotive motor oil. Most motors have lubricating ports – small holes – for applying machine oil. Use only a couple of drops of oil. Avoid the temptation to flood the bearing. If you do, the motor will just throw the excess all over the place.

If a “capacitor start” or a “capacitor start/capacitor run” motor will not start, check the capacitor. When a capacitor fails, the motor may overheat and either will not start or will not run correctly. Capacitors are physically located outside the frame of the motor, and are much less expensive to replace than the motor. This is particularly true if the motor is an air conditioning/heat pump compressor sealed into a refrigerant system.

Brushes wear in normal service and are normal maintenance parts. Replacements are available from the tool or appliance manufacturer. Typically, the motor will show symptoms of impending failure. Brushes wear in operation to the point where they no longer make good electrical contact. Often, a small external physical bump will cause the motor to start. That’s a sure sign that the brushes need replacing. Order replacement brushes when symptoms first appear, or the tool will surely fail when you most need it, before replacement brushes are on-hand.

Motors are very reliable devices. Motors will generally give many years of satisfactory performance. The down side of that is that your specific model may not be available when you do need to replace it. If a motor will not start due to internal failure, you do have options. I recently had occasion to help a friend with a blower motor for his onboard air conditioning unit. The manufacturer wanted over $400 for a replacement blower. Instead, we took the motor to a local motor refurbisher, and for $60, the refurbisher replaced the bearings and rebuilt the motor. Centrifugal switches are also replaceable. Electric motor refurbishers are available in most medium sized and larger communities across the country. Don’t overlook this option. Look under “Electric Motor – Repair” in the Yellow Pages! Yes, folks, I grew up using Yellow Pages.

Refrigeration compressors have built-in safety circuits. One is a thermally operated switch that’s mounted to the case of the compressor. It is designed to open and disconnect power to the compressor motor if the compressor case gets too hot. Another is a pressure operated switch that is designed to disconnect the motor if the refrigerant gas pressure gets too high. Some units can also detect low refrigerant pressures. These switches can fail, and their failure rate is higher than the failure rate of the compressor itself. If a compressor fails to run, check the safety switches before changing the compressor or changing the entire fridge or air conditioner/heat pump unit. Many an unsuspecting soul has paid to have a compressor replaced and only gotten a $20 switch for the money!

Qualifications of Personnel

The above discussions illustrate an important safety consideration which I know some will find restrictive and controversial. Simply stated, people who are not thoroughly familiar with marine electrical standards and requirements should not install or modify boat electrical systems! Many excellent residential electricians and many skilled DIY “practitioners” who learned terrestrial NEC compliance techniques in residential applications are simply not qualified to perform work on boats. Switching requirements are different on boats than they are on land, yet it is true that cheaper switches incorrectly selected for use on a boat may appear to work. The details of neutral-to-ground bonding are much more extensive on boats, yet man-made wiring errors may go hidden and without symptom for weeks, months or years. Work performed by one who is simply unaware of boat equipment requirements can lead to unintended but serious safety faults for friends and family to discover at some random future time.

The frustration of encountering a no-power situation because the boat trips a ground fault sensing pedestal breaker on a cruise is unwelcome for the boat owner, but is truly unsettling to the spouse and guests aboard. Diagnosing man-made wiring errors is expensive and frustrating by any definition. It is extremely important to know, understand and comply with the low-level details of the ABYC electrical standards. Boats in marinas are in very close proximity to their dock neighbors. All marina residents – whether longterm or transient – depend on the safety of neighboring boats. When hiring someone to do electrical work on your boat, make sure the person you hire is actually qualified by training and certification to perform marine installation, maintenance, troubleshooting and repair services.

Incidental Topic – Dockside Ground Fault Sensors

While not actually a boat-side AC electrical topic, GFIs on docks is a topic that does apply to any discussion of boat AC electrical systems. The problems that cause dockside ground fault sensors to trip are all caused by conditions that exist on the boat. Many (the great majority) of these issues were caused by unqualified but well-intended DIY practitioners who did the wrong things without realizing it. I have written in detail about dockside GFI problems and solutions. Articles on this website that discuss these issues include:

  1. Electric Shock Drowning
  2. Emerging AC Electrical Concern
  3. AC Safety Tests for Boats
  4. ELCI Primer
  5. Ground Faults and Ground Fault Sensors
  6. Ground Faults: Difficult to Hire Skilled Troubleshooter

Incidental Topic – Galvanic Corrosion

Also not an AC electrical topic, this heading is included because the Galvanic Isolator
is fit into the main safety ground conductor of the boat. The submerged metal parts of boats are comprised of a mix of dissimilar types of metals. Boats commonly have
stainless steel drive shafts and rudders, bronze propellers, struts, rudders and thruhulls, and Aluminum trim-tabs. When immersed in sea water, these different metals and metal alloys follow the same laws of electrochemistry as found in a battery, albeit not optimized in construction and materials purity as they would be in a made-for-purpose battery. The action of this electrochemistry results in “metal wasting” corrosion of some of the underwater metals.

Another very common form of galvanic corrosion is “single-metal” corrosion (ex: “rust”
in iron-containing metals, “poultice corrosion” in aluminum, “pitting corrosion” and
“Crevice corrosion” in Stainless Steels). A serious and often unrecognized form of
single-metal corrosion occurs in the all too common brass plumbing fittings bought in
big box and hardware stores, and even in some marine chandleries. Brass is a metal
alloy containing primarily copper and zinc. We know zinc is a galvanically active metal
(anodic) that will sacrifice itself to protect more noble metals (cathodic). Brass fittings
flooded in sea water suffer from a phenomena called “Dezincification.” The zinc
wastes away, leaving the remaining metal structure of the brass alloy porous, with a
pink appearance, and physically very weak.  WARNING: never use brass fittings
below the waterline or in raw water circuits used by heat pumps aboard the boat.
“Sacrificial anodes” of zinc, aluminum and magnesium are usually attached to valuable underwater metals to protect the more valuable metals from galvanic corrosion wasting damage. Zincs are most effective if electrically located on the metals they protect.  Zincs waste away as they give up positive ions to the electrolyte of the galvanic cell.An “Impressed Current Cathodic Protection” (ICCP) device is an electronic approach to
managing galvanic corrosion on boats built with metal hulls (steel, aluminum). An
ICCP is able to protect the relatively very larger surface areas of metal hulls than can
be done effectively with individual sacrificial anodes.

About – Galvanic Isolation

The ABYC recommends some form of galvanic corrosion control on boats. Aside from
the active electronics of an ICCP, there are three passive ways to achieve this control.
One modifies the electrical makeup of the underwater collection of metals. The other
two act by disrupting the flow of the small but destructive DC galvanic currents. The
latter two approaches impact upon the design of the boat’s shore power safety ground.

The first and most common approach to reduce galvanic wasting is with the use of
sacrificial anodes. These anodes modify the makeup of the underwater metals in a
way that makes them waste, rather than more valuable metals.

The second approach is with the use if a Galvanic Isolator (GI), which eliminates the
electrical path for galvanic currents to use. Electrically, this device is placed in the
main safety ground wire where the ground conductor enters/exits the boat; that is,
electrically at the shore power inlet(s). The newest generation of GI is the “Fail Safe”
device. It consists of a solid state, full wave, bridge rectifier and a large capacitor. This device will allow AC fault currents to flow normally in the safety ground, should
that need ever arise. The physics of the diode junction effectively blocks the small DC
voltage that drives the flow of galvanic currents. Without a galvanic isolator, zincs can
be consumed in weeks. With a galvanic isolator, zincs should last many months.

The third approach to interrupting the flow of galvanic currents is by installing an
onboard AC “shore power transformer.” An Isolation configuration eliminates the path
from the boat’s grounding network to shore. A polarization configuration keeps the
shore path, so should include a Galvanic Isolator. There are subtle pros and cons to
this choice. This author prefers the polarization configuration for maximum safety.

Electrical Emergencies

True electrical emergencies are rare. Electrical emergency situations will always
become less dangerous if power is quickly disconnected.

Be wary and suspicious of unfamiliar, unpleasant or pungent odors. Transformers,
motors and many other electrical devices that are in the process of failing often
overheat and cause insulating materials to emit strong, pungent odors. TURN OFF
POWER and use your nose to track down the source. Turning power off will also
shut down air circulating blowers that circulate odors and make locating their origin
difficult. Treat strong odors as an pre-emergent true emergency. The goal is to
find the offending device before it ignites! Turning off the power will stop the self destructive process and allow the failing device to cool off. Do not re-start a device
that has overheated in operation to the point of emitting strong odors! This type of
over-heating often causes secondary internal damage that you cannot see.

In an emergency, the most important commodity you can have is time! Time to
think and act. To buy time, install smoke detectors. Install smoke detectors that
have dual mode incipient gas sensors as well as visible smoke sensors. Install a
model that communicates with other units so that when one alarms, they all alarm. I
placed a dual-mode smoke detector on the overhead of my electrical locker. That
locker is a small, closed space behind my AC and DC branch circuit panels, and is
where the shore power inlets and the Generator Transfer Switch are located. That is a
good place to install a smoke detector, placed there in order to buy me some time.

Emergencies – Avoidance

When working around electricity, use insulated tools, especially when working around batteries. Batteries contain enormous amounts of stored energy. A metal tool across the terminals of a battery may actually weld the tool metal to the battery terminals. If this happens, the tool metal will become extremely hot. Whenever you plan to work around a battery, pre-plan to have a two foot piece of 2”x2” wood stock, or a wood handled carpenter’s hammer, readily at hand. If the worst should happen, use the wooden 2”x4” as a mallet to forcefully knock the tool away from the battery terminals. Once this cascade of events starts, the only way to stop it is to break the tool free of the battery terminals. Act quickly. The battery can get hot enough to melt and start a fire.

Many electrical emergencies are avoidable. Always comply with standard electrical
safety rules and practices. This is not a exhaustive list. As you plan your projects,
plan for safety.

    1. Never work on live electrical circuits. Turn power “off” before accessing.
    2. Never work alone; always have someone with you who can disconnect power
      and call for help in an emergency.
    3. Never wear watches or jewelry when performing electrical work.
    4. Never parallel multiple small gauge wires to achieve a larger current carrying
      capacity (“ampacity”). That virtually guarantees trouble in the future.
    5. Install protective insulation and safety covers to prevent accidental contact with
      bare electrical connections and terminals.
    6. Periodically, go on an “inspection tour” of the boat’s electrical system; make this
      a part of your scheduled preventive maintenance checklist. Specifically,
  • Screws work loose over time; with power off, periodically go through the boat
    and tighten electrical connections.
  • Crimp connections corrode and loosen over time; avoid crimp connections
    wherever possible; given the choice to splice an existing wire or run a new
    wire, run the new wire; with power off, check crimps by firmly pulling on the
    wire at the crimp. Replace any connections that show any signs of heating
    or of being or becoming loose!
  • Secure loose or dangling wires.
  • Check wiring bundles where they ride over or round obstructions or through
    bulkheads. Vibration injures insulation and wiring, so support and insulate
    bundles in these areas to prevent wear spots.
  • Leave adequate slack in wire runs so they are not under tension.
  • Repair cuts, cracks or gouges in insulation immediately. Don’t wait.

In Case Of Fire

“Experts” all agree, in any fire on a boat, 1) there is very little time to act, and 2)
the odds of successfully fighting a fire are against you from the beginning.

If there is any doubt about being successful at extinguishing a fire aboard, use the
precious little time available to get your crew and yourself safely away from the fire.

    1. Alert your crew:
      • If you decide to fight a fire, do not use water! Water can conduct electricity,
        and you may wind up with both fire and electrocution emergencies. To fight
        an electrical fire, use a dry-chemical extinguisher rated for “Type ABC” fires.
      • Crew calls “m’aidez” (“May Day”) via VHF-16, or 911 via telephone. Do not
        hang up the phone until the 911 dispatcher tells you to.
    2. Disconnect Power:
      • If on shore power, turn power “off” at the pedestal!
      • If on genset, shut down the machine!
      • Shut down DC power to any inverter or inverter/charger!
      • Disconnect the main battery bank!
    3. Once the fire is extinguished, monitor the involved area to be sure it’s cool
      enough that it will not self re-ignite.
    4. Make repairs before re-applying power.

Appendix 1

The following is more in depth than I usually write, and will be of interest to advanced
DIY practitioners and electrical professionals interested in how electrical safety and
testing codes are applied. This material adds to what has been presented above, but
is not necessary to understanding.

Acronyms and Abbreviations

ANSI – American National Standards Institute
AHJ – Authority Having Jurisdiction
CSA – Canadian Standards Association
DER – Distributed Energy Resources
DOE – United States Department of Energy
EPS – Electric Power System
ETL – Intertek® registered testing mark (Electrical Testing Laboratories)
IEC – International Electrotechnical Commission
IEEE – Institute of Electrical and Electronics Engineers
NEC – National Electric Code (United States)
NREL – National Renewable Energy Laboratory
PUC – Public Utility Commission
REPC – Rural Electric Power Conference
SGIRM – Smart Grid Interoperability Reference Model
UL/ULc – Underwriters Laboratories® testing mark

ABYC A-32, AC Power Conversion Equipment and Systems

All ABYC Standards follow a common layout format (“boilerplate”). Following is an
excerpt from the “References” section of ABYC A-32, July, 2017:
32.3 – References
The following references form a part of this standard. Unless otherwise noted the
latest version of the referenced standard shall apply.
32.3.1 – refers to several other ABYC standards
32.3.2 – IEC 62116 Test procedure of islanding protection measures for utility interconnected photovoltiac inverters (IEC 62116 is a European standard)
32.3.3 – IEEE 1547 Standard for Interconnecting Distributed Resources with the
Electric Power Grid (IEEE 1547 is a US ANSI Standard (North America power grid))

Author’s note: emphasis and comments added for clarification.

Relationship of IEEE 1547 and UL 1741

Safety Standards define minimum feature and function capabilities for the design of a
particular class of equipment; in this case an inverter-charger DER. Testing Standards
define test specifications that a device must meet in order for the manufacturer to claim
compliance to the design standard. This leads to some very complex relationships
between different national regulatory authorities and between and among multiple
independent, private enterprise businesses. Following is a pictorial that shows the
relationship of the safety and testing standards that define the INTERFACE between
devices in the class of DERs to the North American Electric Power Grid as deployed in
the United States:

In the above Figure 6, the IEEE 1547 Standard defines the minimum design
requirements of DER equipment. IEEE 1547.1 and UL 1741 together define the
minimum test conditions that the completed device must meet. In addition, ABYC A32,
32.9.2 calls for disconnect protection in less than 100 mS after loss of incoming AC
power. The NEC, Article 705, defines what the National Electric Power Grid is


Figure 7 shows the Certificate of Conformity for the Victron® MultiPlus™ device family.  At the bottom of the Certificate, readers can see that the device complies to UL 1741-2016 (2nd Edition) and the Canadian National Standard, CAN/CSA 22.2, No. 107.1-16, (4th Edition).


  1. Victron® MultiPlus™ and Quattro™ inverter-chargers are grid-attached DERs,
    even though their purpose when installed on boats is not to supply power
    backwards onto the grid.
  2. ABYC Standard A-32 incorporates the requirements of IEEE1547. All of the
    ABYC standards operate in the same way, by including (incorporating) other
    relevant IEC, EN and IEEE standards into themselves.
  3. IEEE 1547 has been adopted as an American National Standard by the
    American National Standards Institute. IEEE1547 and subs (1547.1 through
    1547.8) state the design and testing requirements that DERs used in the US
    must meet; in this case, we are specifically interested in the Victron® MultiPlus™
    and Quattro™ inverter/charger devices. Victron® complies to UL1741, which is
    compatible with the NEC in the US.
  4. At this writing, I am still investigating, but I believe it is true that when UL 1741 applies to a device, that certification supersedes UL 458. UL458 compliant
    devices disconnect the incoming mains when the device is operating in “invert”
    mode. UL1741 compliant devices do the same thing, but for a broader set of
  5. Per Victron®*, “…we disconnect/get isolated from the AC source within 20mS.”
    That is well within the July 2017, ABYC A-32 requirement of 100mS.

* email to the author dated 3/15/2019, signed:

Mr. Justin Larrabee
Sales Manager
Victron Energy
70 Water Street
Thomaston, ME 04861

AC Electricity Fundamentals – Part 1

2/14/2019:  Initial post
6/6/2020:    Added: “US Utility Voltage Standards vs. Common Language”

About this article

This article discusses the concepts and terminology of AC electricity at an introductory level. The scope of the article is limited to the AC power systems found in North and Central America. In Part 1 (this part; already plenty long enough), I will discuss the basics of AC power generation and the delivery of AC power to single-family residential neighborhoods and homes. In the Part 2 article,  I present a discussion of the AC power systems focused on cruising boats.

I chose this two part approach for two reasons. First, almost all homeowners have some familiarity with household AC electricity. At the very least, most homeowners can find the circuit breaker panel and reset tripped breakers. Second, and more important, boat AC electrical systems are just a specific subset of what is found in a single-family residential AC installation. Boat AC systems are equivalent to sub-panels in a residence. Sub-panels are subordinate to the main service disconnect panel in a residential building, and in the same way, boats are subordinate to the AC electrical infrastructure of a marina. A basic understanding of household AC electrical systems puts boaters 75% of the way towards understanding boat AC electrical systems. Where boats differ from land-based residential buildings, the reasons are based on specific safety issues that emerge in, and are unique to, the marine environment. Boat AC electrical systems are significantly more complex than single family residences.

This article will assist readers in having confidence when talking about electrical topics with a professional, marine-certified, electrical technician, either designer or tradesman.


There is one absolute, always rule whenever you must deal with electricity. VIRTUALLY ALL ELECTRICIY CAN BE DANGEROUS TO PROPERTY AND LIFE. Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.  The large batteries found on boats can produce explosive gasses and store enough energy to easily start a large, damaging fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity! If you will be working in noisy environments, with running engines or other loud machinery, WEAR HEARING PROTECTION.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right tools for a job…
If you are not sure you know how to use the tools you do have…
Well, then, LEAVE IT ALONE until you learn more!

The rule is, “if you aren’t sure what to do and how to do it, stop. Don’t do anything until you’re sure of the “what,” “how” and “why!”


Electrocution is a biological insult that starts with an electric shock that paralyzes either the respiratory or cardiac functions of the body, or both. Electrocution results in death.  Even very small electric currents, under the right circumstances, can result in electrocution. Obviously, electric shock can be a life threatening emergency.

If you are present and witness an electrocution, there are several things to do immediately. Remember, since the victim is not breathing, you’ll have 5 minutes or less to accomplish items 3 – 10, below:

  1. STAY CALM!  You can not save someone else if you panic!
  3. SCREAM FOR HELP! ATTRACT ATTENTION!  Point at the first person who’s attention you get and instruct them to “call 911 for an electrocution!”
  5. If the victim is in the water, KILL POWER TO THE ENTIRE DOCK.
  7. After power is removed, raise the face of an unconscious victim out of the water.
  8. After power is removed and the victim’s airway is secured above water, if help has not arrived, call 911 again!  Two 911 calls are better than none.
  9. After power is removed, and with access to the victim, assess victim and initiate CPR as appropriate.  CPR is often successful in reviving or saving electrocution victims who are otherwise healthy at the time of the accident.

Basic Electrical Working Concepts  – Volts/Amps/Ohms

Like gravity, electricity is invisible. A common analogy used to explain electrical concepts is to liken an electric system to a community water system. Consider the familiar garden hose fit with a nozzle. In the garden hose, when the nozzle is opened, “pressure” in the system makes water flow.

In this analogy, the water in the hose is analogous to electrons in a wire. “Voltage” is the “propulsive energy,” or “pressure,” that makes electrons flow through the wire. The greater the water pressure, the more water flows per unit time. Similarly, the more voltage that is present across a circuit, the more electrons will flow through the circuit per unit time. The amount of water that comes out of the hose is measured in “gallons.” The flow of electrons through wire is measured in “amperes,” or “amps.”

In a water hose, the nozzle restricts the flow of water through the hose. The flow of electrons is restricted in electrical circuits by the electrical property of “resistance.” All materials that conduct electricity have some amount of resistance. Silver and gold have little resistance per unit length. Pure copper has only slightly more, and aluminum has slightly more again. Even the small resistance of a copper wire is extremely important in power distribution applications. Electrical “resistance” is measured in “Ohms.”

Assume we have a 3” diameter water hose and a 1/2” diameter water hose, both attached to the same water source. Only so many molecules of water can fit through the small hose in a minute, but many more molecules of water can fit through the large hose. This concept is called “carrying capacity.” Only so many electrons can “fit” through a wire per unit time.  The larger the wire, the more electrons.  Electrical “carrying capacity” is called “ampacity.” “Ampacity” is a rating assigned to wires.  Wires of the same metal, of different sizes and covered by insulation with different thermal and chemical properties, have different rated “ampacities.” The ampacity rating is the safe maximum current the wire can carry within the temperature rating of the wire’s insulation. Ampacity tables are widely available on the Internet.

Ohm’s Law – Memory Aid

The mathematical relationship between voltage, current, resistance and power is defined by “Ohm’s Law.” Ohm’s Law is probably the most fundamental relationship there is in the entire realm of electricity.  Folks who deal with electricity regularly have this relationship emblazoned in their brains, but for the rest of us, this “memory aid” is  extremely helpful! First, decide what variable you want to calculate. It’s unusual not to know at least two of the necessary variables. For example, today I saw a TV advertisement for a small, portable, plug-in electric space heater. The device plugs into a 120V outlet, so we know E = 120V. I went to the website and found that the unit is rated at 600 Watts, so we know P = 600. For use on the boat, I wondered how much current the device would draw. From the two known variables, we can calculate that the unit will draw about 5 Amps of AC current, which indeed may be OK for some uses on a boat. We also know the unit’s equivalent resistance is 24 Ω (“Ω” is the Greek Letter “Omega,” and is used as shorthand for “Ohms.”)


The Earth – the crust of our beloved home planet – is electrically conductive. It has many minerals and mineral salts which provide “free electrons.” In the presence of a voltage, electrons flow from point-to-point around and within the earth’s crust. By far the most dramatic example of this is the natural phenomena called “lightening.”

The electrical potential of the earth is defined to be “zero” volts. It is the standard reference point for shock and electrocution safety. In order to connect a residential electrical system to “earth ground,” one or more interconnected rods of copper are driven into the ground. The neutral return point of the residence’s electrical system is physically connected to the network of copper grounding rods.

The concept of “earth ground” is absolutely essential for the safety of people, pets, farm animals and wildlife.  The entire electric distribution grid of the country is connected at innumerable points to rods driven into the earth (the “electric grid” is a “multi-earthed system”).  Every residential property has an “earthing” connection at the service entrance to the home.

The essential point here is that “earth ground” is a universally understood reference point for all power distribution systems. It represents the presence of “zero” electrical potential, or stated in the negative, the total absence of any voltage.  We will return to this concept over and over as we proceed in our discussion.

Circuit Common/“Common”

The concept of “earth ground” is essential for electrical safety, but an earth ground is not necessary for electric circuits to operate. The term “common” is useful in electrical design. It is used among power distribution engineers and craftsmen to reference the conductor that returns current flowing in a circuit from the load to the source.  This is the purpose of the “neutral conductor” in AC electric systems. This conductor does not have to be “0” volts with respect to ground. The “common return” is a “free-floating” conductor. It is extremely important to understand the difference between the concepts of “ground” and “common.”

The term “common” or “circuit common” is not often used in routine conversation.   The common return of a circuit is frequently – colloquially – called its “ground.”  The most appropriate term in household electrical systems is “neutral.”  “Neutral” is a specific term that refers to the current-carrying return conductor of residential AC circuits, but it is not a specific reference to “earth ground.”

Direct contact with energized high voltage is completely safe as long as you are not “across” two or more electrical conductors. For current to flow, there must be a connection between two conductors where there is a voltage difference between them (that is, “across a voltage”). Consider, birds sitting on high tension transmission lines, or squirrels running along neighborhood overhead wires. They are safe because they are on, but not across, a voltage. The animal’s entire little body is raised to the voltage of the wire upon which they sit, yet they are perfectly safe because there is no path for current to flow THROUGH the body. The electrical activity of their brains and hearts is not affected. But, a human being on a wet concrete floor wearing leather shoes best not come into contact with a “hot” wire. That concrete floor is made with salt-containing minerals, and most definitely is electrically conductive, especially when wet. A person standing on that floor and simultaneously touching an energized wire is “across” an electric voltage. That is a shocking experience!  Maybe, a fatal, shocking experience.

“Conventions” vs. Facts

Within the study of electricity as a science, there are hard electrochemical and materials facts, and then there are shorthand ways people talk to each other about complex concepts.  This happens in all professions, of course.  It’s all fine until the terminology confuses an understanding of the true concepts.  Some examples:

  1. It is a fact of physics that electrons carry a negative electrical charge, which means electrons flow from a more negative voltage in a circuit to a more positive voltage.  However, by universal agreement, or “by convention,” the entire practice of electricity and electronics treats current as flowing from positive to negative.  The direction of electron flow has no practical importance, but to properly interpret electrical diagrams, you need to understand the conventional way current flow gets represented by arrow-containing symbols.
  2. The symbols on electrical drawings are all agreed by “convention,” or “working agreement.”  Industry-specific symbols are agreed by international standards organizations.  Where there are symbol differences, their meaning is often obvious.  Some differences occur across international boundaries.  The power industry uses different symbols than are used in the electronics industry.
  3. The “single phase, center tapped, three wire” service is the residential standard in use, by convention, all across North and Central America.  It is institutionalized in the National Electric Code of the US and The Canadian Electric Code in Canada.  Completely different systems are used in other parts of the world, including Europe, Asia, Oceania and southern South America.
  4. The insulation used to coat electrical conductors is colored.  The colors, by convention, identify the use to which wires are put.  Understanding the color schema for wires is essential to electrical safety.  Mistakes here can be fatal.  The meaning of colors vary from country to country.  There are numerous differences between the United States and the nations of the European Economic Community and Oceania.  For those interested, tables are available on the Internet that document color meanings.

Science and Craftsmanship

The laboratory study of “electrical energy” is a theoretical and conceptual science.
Electrical craftsmanship is practical.  I will discuss only a tiny subset of the technical terms and concepts that are necessary to understanding low voltage AC as found in residential and boat applications.  Craftsmanship involves selecting materials, employing fabrication techniques, installing and maintaining electrical equipment, all with the goal of accomplishing some intended design purpose.  Craftsmanship is performed by electricians or electrical technicians and governed by formal regulatory controls called “electrical building codes.”

I view craftsmanship in two stages, which can be sequential or iterative.  If you have ever done an electrical project, you’ve performed both of these functions.

The first stage is the domain of the “circuit designer;”  i.e., the person who designs a branch circuit for installing a ceiling fan with a single switch to turn the fan “on” and “off.”  Or a slightly more complex branch circuit with three switches to turn a light “on” and “off” from different locations.  Or a much more complex array of multiple branch circuits to power a “man cave” or “she shed.”  Or the system for an entire home.  The designer must have solid knowledge of the National Electrical Code (NEC).  Electrical designers for boating applications must be thoroughly familiar with the American Boat and Yacht Council (ABYC) electrical standards.  The NEC and ABYC standards have as their purpose avoiding or minimizing present and future loss of life or damages to property.  The work product of the designer is a system drawing that defines the purpose of a circuit and the manner in which that purpose will be achieved through the use of electrical equipment, components and materials.  The work product includes a the bill-of-materials of the components required to implement the project.  For most projects, a reliable cost estimate can be produced at this stage.

The second craftsmanship stage is the domain of the skilled technician who is charged with the doing of the thing.  This craftsman must know how to use and interpret the designer’s drawings and how to use an enormous array of electrical meters and mechanical tools in the safe fabrication, construction, installation and maintenance of electrical circuits.  This craftsman must understand current assembly techniques, materials and supplies, and must understand and deeply respect industry safety practices.  Safety practice involves knowing when to and when not to work around, and with, energized electrical circuits.  On boats, because of the special safety implications of an electrical system on a floating structure, this craftsman must understand not only what to do and how to do it, but in fact, why things are done as they are, in making an electrical installation safe.

Key Concepts and Terms

  1. Ohm’s law – describes the mathematical relationship between voltage, current, resistance and power.
  2. voltage – (Volt) the quantification of “Electromotive Force” (EMF) (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive Force is measured across two points in a circuit.
  3. current – (ampere; amp) a quantification of the number of electrons flowing through a circuit at any one time.
  4. resistance – (Ohm) a characteristic of an electrically conductive material that tends to retard or impede the flow of electrons through it.
  5. power – (elect: Watt; Joule) (mechanical: inch-pounds, foot-pounds) elect: the amount of “work” that electricity performs in its application.  In purely resistive applications, light or heat.  In turning a motor, torque.
  6. frequency – (Hertz) the number of times a wave goes through a complete cycle in a standard measurement time interval, usually one second.
  7. ampacity – (Amp) a rating of the ability of a conductor of given material, diameter and insulation properties to conduct an electric current within the temperature limit established by the properties of the wire’s insulation characteristics.  (current: amperes; amps) (temperature: degrees Centigrade)
  8. source – the origin from which AC power emerges to energize a circuit.
  9. load – the components of an electric circuit where energy is consumed to do useful work; “useful work” includes production of heat, light, or torque via a motor.
  10. common – a portion of a circuit connection or set of connections that creates a direct return path for electrons flowing in an electric circuit.
  11. neutral – a special case in an AC circuit of a non-ground return path for electrons flowing in a North American standard residential electrical service.
  12. ground – a universal  standard earth reference voltage of “0” volts.
  13. fault current – an abnormal path for current flow, usually to ground.  Fault currents represent potentially dangerous conditions.
  14. short circuit – a specific category of electrical fault resulting from an unintentional direct connection of an energized conductor to either a return circuit or an earth ground.  This low-resistance, unintentional connection results in the flow of extremely large fault currents, and causes overload protection devices (fuses, circuit breakers) to open in order to disconnect the energized power source.
  15. GFCI (Ground Fault Circuit Interrupter) – an anti-shock safety device that senses leakage currents and disconnects the energized power source.
  16. AFCI (Arc Fault Circuit Interrupter) – a fire protection safety device that senses lose connections and disconnects the energized power source.
  17. GFP/EPD/ELCI (Ground Fault Protection/Equipment Protective Device/Equipment Leakage Circuit Interrupter) – similar to GFCI, but higher disconnect specifications.
  18. chase, raceway, conduit, “emt” – enclosed containment spaces in a building or a boat through which wires are run to achieve access to distant locations or to protect wiring from accidental physical damage.
  19. Field Coil – the rotating part of one design of AC generator; this coil can be a DC permanent magnet (typical in small machines), or a DC electromagnet.
  20. Voltage Regulator – the device that determines the strength of the magnetic field in an AC genset by adjusting DC current flowing in the spinning field coil.
  21. Stator – the fixed coils of one design of AC generator, from which sine waves of AC power emerge.
  22. Armature – the power-producing component of a generator; the rotating part of a DC generator; the fixed coils (Stator) in one design of AC generator.
  23. switchgear – a generic term for all disconnecting devices (fuses, circuit breakers, switches, power panels).  This term is used across the electrical power industry, from generating station to transformer yards to residential locations.
  24. Inductance (Ohm)/capacitance (Farad)/Power Factor (unitless) – technical characteristics common to the behavior of AC electricity in circuits that significantly affect large motor driven appliances and all electronic devices.  These become increasingly important as voltages, frequencies and power consumption rise.
  25. Managing the collapse of a magnetic field – a design consideration of any magnetically operated electrical devices (motor, generator, relay, etc), and many solid state devices.  A significant safety consideration for maintenance craftsmen.  When a magnetic field collapses, it creates a very high energy spike, which sometimes includes an electric arc.
  26. ABYC – American Boat and Yacht Council, Annapolis, MD.  This organization produces a very comprehensive set of electrical standards applicable to boat manufacturers, the marine insurance industry and boat owners.
  27. NFPA – National Fire Protection Association; owner/creator of the NEC.
  28. NEC/CEC – National Electric Code (USA), Canada Electric Code (Canada).  electrical design standards for political subdivisions and the construction industry.  Ranges from codes for residential housing, light commercial and industrial buildings, elevators, hospitals, airports, and heavy industry.

Generation (Source) and Consumption (Load)

There are three primary divisions of all electrical power distribution systems, including the global system we call the “nationwide electrical power grid.”  They are 1) the source of the electrical power, 2) the transmission system, or interconnecting wires and switches that carry power from the source to the point where it is consumed, and 3) the load, or the part of the system where the electrical energy is transformed into useful work.

At the level of the US national electric power grid, the source of AC electric power is one or more generating machines located in one or more generating stations.  Often, the term “alternator” is used interchangeably with the term “generator.”  These generating stations range in size from enormous, industrial-sized installations to small rural hydroelectric dams to units suitable for individual residential applications.

The substations, switchgear and wiring that connects sources of power to load centers are extremely complex, involving may hundreds of miles of high tension power lines, enormous transformers and highly complex switches.  Transmission equipment  can also be as simple as an extension cord run from the garage to the hedge clipper.

Electrical loads fall into the entire range of electrical equipment, from the largest commercial synchronous motors to the smallest and most humble LED electric clock.

About AC Generators

Typical AC electric generators have a rotating magnet (imagine the big bar magnet you played with in grade school science) which has a north pole and a south pole.  That magnet may be driven by a belt, wind or water turbine or direct drive, but ultimately, it’s a spinning magnet mounted on a shaft.  The north and south poles of the spinning magnet travel in a circular path.  A pick-up coil is positioned just outside the edge of the circle.   As the magnet spins on it’s shaft, the poles of the magnet approach the fixed pick-up coil, producing an electric voltage at the pick-up coil.

As the spinning magnetic pole gets physically nearer to the pick-up coil, the voltage at the pick-up coil gets progressively larger.  Once the magnetic pole moves past and away from the pick-up coil, the voltage at the pick-up coil gets progressively smaller again. When the north and south poles of the magnet are both equally distant from the conductors of the pick-up coil, no voltage is produced at the pick-up coil.

The voltage induced in the pick-up coil by the passage of the north magnetic pole is equal in magnitude and opposite in polarity from the voltage induced by the passage of the south magnetic pole.  One pair of north and south magnetic poles that sequentially rotate past the pick-up coil produce one cycle of AC voltage at the pick-up coil on each revolution. The speed, in revolutions per minute (RPM), of the spinning magnet determines the frequency (Hz) of the generated voltage.  The resulting AC wave form is called a “sine wave,” which is centered around “0” volts.  Sine waves rise and fall in smooth, graceful fashion with no sharp transitions in the shape of the wave.

In the preceding diagram, there is a geometrically balanced arrangement of a spinning magnet and a geometrically balanced arrangement of pick-up coils.  The output power of the generator is directly proportional to the strength of the magnetic field, up to the limits of its materials and mechanical design.  The output consists of two wires, and is referred to as “Single Phase” AC.

Many physical arrangements of the magnet poles and pick-up coils are possible, but the basic principle is the same for all AC generators.  To produce 60Hz AC, a single phase, two-pole, gasoline-driven, big box store genset (2500W to 6kW) typically spins at 3600 rpm; a single phase four-pole Marine genset (7.5kW to 25kW) typically spins at 1800 rpm. Because of the enormous weight and mechanical forces involved, multi-megawatt commercial generators may have 24 poles and spin at 200 rpm.

The rotating magnet in an AC generator is called the “field coil.”  The field coil is just a spinning DC electromagnet.  DC is fed to the field coil via slip rings and brushes on the spinning shaft.  The fixed pick-up coil in an AC generator is called the “stator coil.”  It is wrapped around iron support columns that are fixed in position around the perimeter of the frame of the machine.

Occasionally, the term “armature” may be heard; an “armature” is defined as the power-producing component of a generator.  In a DC machine, it is the armature that spins, with field coils stationary in the frame of the machine.  Fixed field coils with a spinning armature is a construction alternative for small frame AC alternators (<25kW).  This is both more costly to build and much more complex mechanically, so not common in the generator sizes found in the consumer retail market.

The amount of power that a generator can produce depends on many aspects of the physical construction of the machine, the amount of energy available from the driving motive source, the size of the internal conductors and underlying metal components, the strength of the internal magnetic field, and many other factors.

“Single-Phase” and “Three-Phase Power”

In reading through questions and discussions on various Internet boating bulletin boards , the differences between “single phase” AC and “three-phase” AC is often a point of confusion.  Three-phase power is extremely rare in residential settings, and few people have any life experience with it.

Consider the above preceding description of generator concepts.  Commercial power plants are fit with enormously large and heavy generators.  For several reasons, it is advantageous for these very large machines to spin slowly.  These generators are built with a large number of physical pick-up coils.  These pick-up coils are arranged as pairs in sets of three.  Logically – not physically – the machine appears as shown in this diagram.  These sets of pick-up coils are placed around the perimeter of the circle of the rotating magnet, at geometric intervals of 120° around the 360° circle.

As the bar magnet spins, the voltage in each of the pick-up coils rises to its positive maximum, falls back to zero, then rises to its negative maximum, and falls back to zero. This happens in each set of coils, in turn.  The result is three sinusoidal waveforms being produced by the same rotating magnet (“field”).  The three wave forms are displaced in time by 1/3 of a cycle (120°) of rotation of the rotor.  Enter here, the short-cut language the electrical industry has for this: “three-phase AC,” often shown on electrical diagrams written as “3-ϕ” or “3-phase.”

For commercial power plant operators and distributors, three-phase power is far more economical to generate than single-phase power.  Worldwide, all commercial electric power is created in generators configured as 3-ϕ machines.  The phases are designated as “Phase-1,” “Phase-2,” and “Phase 3;” this terminology can also be “Phase-A,” “Phase-B,” and “Phase C.”  Three-phase derived power is of special interest for boaters with 120V/240V, 50A shore power connections since it results in 120V/208V voltages.  More details in the section on “Special Situations.”

Single-phase AC is the type of electric service found in virtually all single family residential applications because it is easily derived from three-phase distribution systems, in two ways.  The first is to connect a load between any one of the phases of a three-phase service and a suitable electrical return point, usually the common of a 3-phase wye configuration.  This is how residential neighborhoods are serviced.  The second way to obtain single phase AC is to connect the load between any two phases of the 3-phase distribution system.  This is common in commercial applications and in apartment and condo buildings, but not in single family residential services.

US Utility Voltage Standards vs. Common Language

In the US (throughout North America), just what voltage standards do we have for residential and light commercial use?  Interesting question.  Is it 110V, 115V or 120V?  Is it 220V, 230V or 240V?  When people speak about residential voltages, it’s quite common to hear one or more of these numbers.  In actuality, they all mean the same thing.

Standardized utility voltages evolved over the years from 110V220V systems to 115/230V to 117V/234V to 120V/240V.  In the 1970’s, the American National Standards Institute (ANSI) adopted the now current 120V/240V voltage standard via ANSI National Standard C84.1-1970.  This standard specifies two voltage ranges which included a specification for “service entrance voltage” and a standard for the voltage that would appear at user attached devices, called “utilization voltage.”   “Service entrance voltage” is measured at the meter, and “utilization voltage” is measured at the terminals attached equipment.  The occurrence of service voltages outside the specified range (brownouts) was intended to be infrequent.

Following is the chart from C84.1.  The Range A service voltage range is plus or minus 5% of nominal. The Range B utilization voltage range is plus 6% to minus 13% of nominal.

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The occurrence of service voltages outside the Range A limits should be infrequent. Household equipment is designed and rated to give fully satisfactory performance throughout this range.

Range B includes voltages above and below the Range A limits that necessarily result from practical design and operating conditions in utility or user systems, or both. Although such conditions are a part of practical operations, they should be limited in extent, infrequent, and of short duration (brownouts). If they occur on a repetitive or sustained basis, corrective measures should be undertaken within a reasonable time to improve voltages to meet Range A requirements.  Household equipment is designed to give acceptable performance in the extremes of this range of utilization voltages, although not necessarily as good performance as in Range A.

Table 1, below, is useful for an understanding of the relationship of power supplied by a power utility and the standards to which household appliances are manufactured.  The “Nominal” column is what we always talk about in common, ordinary discussion.  The “Service” and “Utilization” column are as discussed above.  The “Nameplate” column shows the voltage that will appear on an item you buy, such as refrigerator, dishwasher, washing machine, air conditioner, TV, Stereo, drill press, air compressor, etc.  The “NEMA” column (National Electrical Manufacturers Association) shows the tolerances used by manufacturing designers in creating the devices you buy.  These are all slightly different perspectives on the same thing.

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There are two net messages here.:

  1. The voltage delivered to any residence will vary throughout the day, throughout the day, and throughout the year.
  2. The equipment in the residence is made to tolerate variation in utilization voltage.

This is very much more important to appreciate and understand when the scene shifts to boats in a marina, or more completely, when the scene shifts to boats cruising from place to place with widely different shore power services.

Residential Neighborhood

For simplicity, I started with power as delivered to single family suburban homes excluding light commercial buildings.  Light commercial buildings (condos, townhouses, apartments, offices, stores, and marinas) can all be served with single phase AC electric service, just as single family residences are, but more commonly, they are served by 3-phase utility service.  I will talk about these buildings later in the section on “Special Situations.”

Utility company power transformers have input sides, called the “primary,” and output sides, called the “secondary.”  Physically, both the primary and the secondary coils of the transformer are independent windings of wire wound around an internal metal core.  The windings are electrically isolated from each other; i.e., “insulated” from each other, but are “coupled” to each other by a shared magnetic field.  As the incoming primary voltage rises and falls, the magnetic field in the metal core strengthens and weakens.  As that magnetic field strengthens and weakens, voltage appears at the secondary.

Residential Single Phase :Street” Transformer (Typical)

The “load” for the transformer outside your house usually consists of four or so residential homes.  Throughout North and Central America the transformer is matched to the primary voltage to produce 120V/240V at the secondary.  The utility company transformer reduces the primary voltage to the residential requirement.  The range of transmission system primary voltages in a three-phase grounded wye configuration include; 34,500/19,900 volts; 22,900/13,200 volts; 13,200/7,620 volts; 12,470/7,200 volts; and, 4,160/2,400.  The first number in these number pairs represents the phase-to-phase voltage; the second number represents the phase to neutral voltage.  A single phase primary in a residential neighborhood is most commonly 7,200 volts, measured phase-to-neutral.  In rural residential primaries, 13,200 volts is common.

Transformer coils can be built with one or more “taps” on both the primary and secondary windings (coils).   The secondary winding of a residential power transformer is built with a single tap at the electrical midpoint of the coil.  This configuration is called a “center-tap.”  The three wires that come to a single-family residential  home from the utility pole are the two end-points of the secondary coil and the center-tap.  That center-tap conductor becomes the “neutral” within the building’s distribution wiring.

In the world of the electrical craftsman (electrician), it is desirable practice in a residential building or boat to have about ½ of the total household load attached to each side of the service transformer.  This practice balances the load on the secondary windings of the transformer on the street, and balances the concentration of heat that builds up within the windings and metal core of the transformer.  Transformers are oil cooled, and under heavy loads, they can get very hot.  Thus, balancing heat dissipation is crucially important in periods of very high electrical demand.  Days that are 104°F on the Chesapeake Bay or -30°F at International Falls are not times you’d want the transformer that serves your home to fail!

The definition of a North American residential standard power distribution system is a “single-phase, center-tapped, three-wire” service (alternatively, “single-phase, center-tapped, three-pole” service; in this case, the term “pole” represents a current carrying conductor.  Other common terms for this systems include “grounded neutral” and “split phase.”  The three parts of this definition are:

  1. single phase
  2. center-tap (gives rise to the system “neutral;” “grounded neutral”)
  3. three wires (two “hot” and one “neutral”)

From time-to-time, professional electricians and DIY lay technicians incorrectly refer to the residential “single phase, center tapped, three-wire” configuration as consisting of two phases.   The “evidence” is that one leg, “L1,” is 180° out-of-phase with the other leg, “L2.”  While “true,” this misleading factoid is a measurement curiosity caused by performing the electrical measurement from an inappropriate reference point.  Voltages from the two halves of our residential service will appear to be out-of-phase if measured with an oscilloscope from Neutral to “L1” and then from Neutral to “L2.”  The false appearance is the result of looking at the secondary of the transformer with reference to its center tap rather than across the entire winding.  This measurement curiosity is not present if the secondary is measured from “L1” to “L2” (or vice versa).  Think of it this way.  There is only one magnetic field alternately rising and falling in the transformer, driven by the rise and fall of the single-phase input at the primary.  That is the defining characteristic of “single-phase” equipment.  In a three phase device, there are three independent magnetic fields rising and falling within the equipment.  That is the defining characteristic of 3-phase equipment.  This distinction becomes extremely important when describing rotational torque in a 3-phase motor.

The above discussion is somewhat of a “technicality” issue, which has no practical importance in real life, and can safely be ignored!  When I was a pup, and first worked for an electrician in the early 1960s, I learned to refer to the two residential “hot” lines as “legs” instead of “phases.”  Doing so distinguishes the in-residence wiring from the conductors of the utility distribution system.  Frankly, except for the concepts involved, it’s really not important how you refer to this as long as you don’t let it confuse you!

Service Entrance – Single Family Residence

So now we understand that the electrical service entering a single family residence is a “single-phase, center tap, three-wire” service.  In our single family residence, if there are overhead wires and utility poles in the street, the three wires coming from the transformer are routed to a weather head or anchorage on the home, where they are spliced to wires leading to the electric meter.  In most jurisdictions in the US, the wires coming from the street are owned by the utility company.  The weather head, meter box and the wires from the “street splice” to the meter box are customer-owned.  The meter itself is owned by the utility company.

The customer-owned wire to the electric meter and from the meter to the main disconnect panel inside the building is comprised of two insulated wires (usually black) surrounded by a wrapping of bare wire strands.  This entire cable assembly is insulated as a single triplex unit.  This cable has a flat rectangular cross-section and is known as “Type SE,” or “Service Entrance” cable.  The two hot lines are routed to the input side of the “main” circuit breaker in the main disconnect panel.  The uninsulated neutral wire of the Service Entrance cable is routed to the neutral buss bar in the main panel.  The neutral buss bar is insulated from everything else in the service disconnect box, including the metal of the box enclosure, itself.  If the residence has an underground service, wires from a transformer located on a ground-level concrete pad will all be individually insulated wires rather than a triplex assembly.  They will be routed through underground conduit into the electric meter and then to the service disconnect panel.

The output side of the main circuit breaker in the disconnect panel is attached directly to metal “buss bars,” to which individual branch circuit breakers are fitted.  These buss bars are referred to as “L1” and “L2,” because they are on the overload-protected load side of the panel’s main circuit breaker.  The input side of the main disconnect breaker is referred to as the “Line” side and the output side is referred to as the “Load” side.

What we have not yet discussed is the “safety ground” that is required throughout the residence by the National Electric Code.  This safety ground attaches to every outlet, switch plate, ceiling fan, luminary fixture and appliance in the residence.  In a residential application, there are one or more copper rods driven into the ground outside the building.  The grounding wire is usually of bare #6 or #4 AWG stranded copper wire, and is routed from the buried ground rod(s) to a buss bar located in the service disconnect panel.  That buss bar is physically mounted on, and electrically connected to, the service disconnect panel’s metal box enclosure.  All of the ground wires that come from outlets and appliances everywhere in the building will be routed to this buss bar.

Main Service Disconnect Panel

We know from earlier discussion that the “neutral” in the building is a free-floating return line for power that arrives from the transformer hot lines.  But a free-floating return point is unlikely to be at “zero” volts, which is required to avoid electric shock in the home.  The NEC requires that the neutral line in a residence be bonded to earth ground “at the derived source of the electricity.”  For a home, the “derived source” is defined to be the main service disconnect panel.

In one design of main service disconnect panel, there is a buss bar dedicated to collecting branch circuit neutral conductors and a physically separate buss bar dedicated to collecting safety ground conductors.  In this style panel, there is a screw – usually dyed green in color – in the neutral buss bar. That screw is the “system bonding jumper,” or “bonding screw.”  This design allows the panel to be used either as a main disconnect panel or as a sub-panel.

If the disconnect panel is to be used as the Main Disconnect Panel, the bonding screw must be seated into the panel’s metal enclosure housing to electrically “bond” the “neutral” buss bar to the “safety ground” buss bar.  That screw is not for any mechanical purpose; it is the electrical bridge that make the “neutral” to “earth ground” connection.  THIS IS A CRITICALLY IMPORTANT SAFETY FEATURE.  NEVER OMIT OR REMOVE THE BONDING SCREW!


Sub-panels, a special case of residential switchgear, are used for several reasons:

  1. reduce the number of wire runs from the main service disconnect panel,
  2. manage the round trip length for long branch circuit wiring runs,
  3. manage the number of wires run in hidden chases/raceways/conduits, and
  4. reduce the cost of the installation.

The NEC does not limit the number of sub-panels that may be installed in a residential electrical system. Larger residential systems may have sub-panels located in several places around the home; ex: attached or detached garage, detached “guest quarters,” workshop, greenhouse or yard shed, pool house, Man Cave, She Shed, attic-space mechanical service (air conditioning compressor or attic vent fans), etc.  To install a sub-panel in residential applications, a single, appropriately sized 4-conductor cable, “Type SER,” is run from the main service entrance panel to the sub-panel (red arrow, below).  This 4-wire configuration carries “L1,” “L2,” “N” and “G” to the sub-panel switch box.  Because the sub-panel is subordinate to the main disconnect panel, the neutral-to-ground bonding screw is NEVER used in any sub-panel switch box.  By definition, the sub-panel is not the “source” for these branch circuits.  The main disconnect panel remains the “defined source” of the circuit.

The configuration of sub-panels in a residence is exactly analogous to the configuration of a boat attached to a marina shore power pedestal.  Notice the 240V, 3-pole, 4-wire feeder (red arrow) that connects the residential Main Disconnect Panel to the remote sub-panel.  This feeder is exactly analogous to the 240V/50A shore power cord of a boat.  The sub-panel “feeder cable” is “Type SER.”  It contains three current-carrying conductors and a safety ground.  Rather than the flat, rectangular cross-section of “Type SE,” “Type SER” cable features a round cross-section.  A boat’s “feeder cable” (shore power cord) is “Type SO” or “Type SOW,” which are very flexible cords.  Net: a boat looks like a sub-panel to the marina’s shore power system, and that is why the ABYC electrical standard seems so closely aligned with the requirements of the NEC.  Notice also in the drawing that the sub-panel safety ground leads back to the neutral buss in the main panel.  The neutral-to-ground bond is made only at the “derived source,” which is the Main Disconnect Panel.   Likewise, on a boat connected to shore power, there should never be a neutral-to-ground connection anywhere on the boat.  In both cases, the neutral-to-ground connection is made at the “derived source,” which is the main distribution panel in a residence, analogous to the marina shore power system for a boat.

Note: This main disconnect panel drawing shows a single buss bar which is shared by the neutrals and the grounds of branch circuits.  This arrangement is an NEC-compliant variation in a main disconnect panel.  Many main disconnect panels and all sub-panels will have physically separate busses for the neutrals and the grounds.

Branch Circuits

Branch circuits are where useful work gets done in the home.  There are three use cases:

  1. Between legs “L1” and “L2” alone, without “N,” we can power 240VAC, two-wire (two-pole) appliances; for example, the 240V motor of a deep-well pump, 240V baseboard electric heat radiator(s), or a 240V hot water heater.
  2. With “L1,” “L2” and “N,” we can power 240V, three-pole appliances; these appliances require 240V for some internal functions and 120V for other internal functions; for example, an electric dryer, range cooktop or oven; all of these appliances require 240V for the heating elements, but 120V for the motor and control circuits.  Or, central air conditioning system, which requires 240V for the compressor, but only 120V for the control circuits.
  3. Finally, with either “L1” or “L2” alone, and “N,” we can power the entire panoply of 120V, two-pole household loads; oil or gas furnace, dishwasher, incandescent and florescent lighting, computers, printers, routers, wireless telephones, TVs, VCRs, stereo, refrigerator, freezer, microwave oven, coffee maker, toaster, crock pot, waffle iron, blender, mixer, hair dryer, steam iron, battery chargers, shop tools, CPAP, oxygen concentrator, etc; you get the idea!

Branch circuits originate at a circuit breaker located in either the main service panel or a subordinate sub-panel.  Branch circuits feed either convenience outlets or feed into the attachment enclosure of a permanently installed appliance.  For convenience of installation and maintenance, the individual black, red, white and bare wires of a branch circuit are packaged together within a sheath of plastic outer insulation.  Most residential wire sold in big box, hardware stores and home centers is “Type NM,” meaning “non-metallic.” This is often called “Romex.”  “Type NM” intended to power 120V circuits is called “two-wire with ground,” or “two-pole, three-wire.”  “Type NM” intended to power 240V circuits is called “three-wire with ground,” or “three-pole, four wire.”    Another common residential wire is “Type AC.”  “Type AC” has an armored metallic sheath around the individual colored conductors instead of a plastic outer sheath.  “Type AC” is used for furnace controls for LPG and oil burners, hot water heaters and other appliance in an equipment room or basement, as well as when installed in areas exposed to being physically disturbed or damaged, such as workshops or garages.  Carefully match the wire you buy to the application you have, based on NEC and local electrical codes.

In the U. S., the color of the insulation on individual wires is important; “L1” is black, “L2” is red, “N” is white and “G” is uninsulated copper in convenience and appliance circuits, but can be green or green with a yellow tracer when insulated.

Occasionally, you may encounter a wire in a service disconnect panel or a junction box that has a piece of electrical tape of another color  conspicuously wound around it near its connecting end.  In a residential building, you may see red or black electrical tape wound on a white insulated wire, or you may see a piece or white electrical tape wound on red or black insulated wires.

Do not remove these pieces of tape; they are not an accidental left-over!  It means the installing electrician has “changed” the meaning of the base color of the insulation of the wire.  In residences, the most common place to find it is in wall boxes containing switches that control lighting or fans from multiple doorway locations, or wall boxes at the top and bottom of staircases.   If you ever see this, always triple-verify how the wire is actually being used before proceeding or disturbing the connection.

I have spent a lot of time talking about the current that arrives at the load in one of the energized conductors, “L1” and/or “L2,” and returns to the source in the neutral, “N.”  I have not discussed the use of the green ground wire, “G.”  In a correctly wired, normally operating home or boat AC electrical system, the ground wire should never have any current flowing in it.  The purpose of the safety ground wire is to provide an emergency path for current in order to trip the supplying circuit breaker to remove power from a faulting circuit.  By definition, current flowing in a safety ground is symptomatic of an electrical fault condition.  Fault currents originate from the hot line(s), but return to the source in the safety ground instead of the neutral.  This condition is also known as a “ground fault.”  Never use wire covered with green insulation as a current-carrying conductor.

Circuit Breakers

Contrary to popular belief, circuit breakers/fuses do not protect attached loads!  Circuit breakers do not protect TVs, entertainment systems, computers, microwaves, coffee pots, pumps or compressors.  CIRCUIT BREAKERS/FUSES PROTECT THE POWER-CARRYING WIRING THAT IS HIDDEN IN WALLS AND/OR ENCLOSED IN CHASES, RACEWAYS AND CONDUIT THROUGHOUT YOUR HOME OR BOAT!  They protect the WIRING of your home/boat.  This is a critically key concept.

When wires overheat, their colored insulation can melt, exposing the live conductor.  At that point, energized conductors can touch other now uninsulated conductors, and sparks can fly.  Wires in closed spaces, unusually warm spaces, or chases/raceways/conduits warm up more than wires in un-congested, cool, spaces where there is plenty of air circulation. Overheating softens the insulation.  Wires can get so hot that they will literally melt and can weld themselves together.  This process can cause adjacent nearby wood and composite building materials to burst into flame.  So, circuit breakers protect wires from overload, and therefore, protect the insulation from overheating, melting, failing and causing fires.

There are several common types of circuit breakers, and several manufacturers of circuit breakers and compatible service disconnect panels.  Circuit Breakers for 120V circuits are singe-wide; for 240VAC, they are “stacked” or “doublewide.”  Doublewide breakers have mechanically linked operating levers, and must be doublewide so that they can be physically installed in a service panel in a way that allows them to mate to both the “L1” and the “L2” buss bars at the same time.   If one leg of a 240V circuit – say, “L1” – develops a fault that causes the circuit breaker to trip, the mechanical link causes the other leg – in this example, “L2” – to also be disconnected from it’s source.  Never remove the mechanical linkage between doublewide breaker operating levers.

Switchgear on Boats – Residential vs. Marine-certified

Circuit Breakers should be selected based on the size of the wire they protect.  A 15A circuit breaker protects #14 AWG, Type NM cable; a 20A breaker protects #12 AWG Type NM, and a 30A breaker protects#10 AWG Type NM.  These numbers are based on the 60℃ temperature rating of “Type NM” wire.  Wire ampacities are higher with the 105℃ temperature rating of “Type BC5W2” boat cable.

Circuit breakers used for “over-current protection” (OCP) have rating of 15A, 20A, 30A or 50A.  That said, modern, sophisticated circuit breakers actually carry several ratings.  In a true short circuit, an over-current fault can instantaneously be as high as several hundreds of amps.  By arcing, that extreme amount of current can weld the contacts closed and permanently damage the circuit breaker’s contact points, rendering the breaker inoperable.  Circuit breakers and all switching devices carry an “Ampere Interrupt Capacity” (AIC) rating.  AIC is the amount of current the device can interrupt without being damaged by arcing.

Modern circuit breakers can also have multiple purposes.  Besides OCP, one added purpose is “Ground Fault Protection” (GFP) and another purpose is “Arc Fault Protection” (AFP).  GFP breakers contain a circuit that compares the amount of current being delivered in the hot wire(s) to the amount of current returning in the neutral.  Any difference in outgoing and returning current is a “ground fault.”  Household “Ground Fault Circuit Interrupter” (GFCI) breakers are designed to trip “off” if the difference between supplied and returned current is as little as 4mA – 6mA.  “Equipment Leakage Circuit Interrupters” (ELCI) onboard boats – and Equipment Protective Devices (EPD) on dockside pedestals – protect the whole boat, as a sub-panel.  ELCI/EPD are designed to trip “off” in less than 100 mS if the difference between supplied and returning current exceeds 30mA.

Finally, for use on gasoline powered boats and environments of potentially explosive gas, circuit breakers (and other electrical switching devices) must be rated as “ignition protected.”  This means that any internal arcing (sparking) caused by the contacts opening under load must not be able to come into contact with any airspace outside the breaker’s enclosure.  If explosive gasses were able to infiltrate the breaker’s enclosure, the vapors would be able to cause an explosion.  Of course, common residential circuit breakers are not made to the standard of “ignition protected” devices.

In general, in my opinion, it is bad practice to use “big box” and hardware store electrical switchgear equipment, circuit breakers or wire made for residential applications on a boat.  Residential switchgear is not made to withstand humid, salt-containing air, is not suited to the materials properties required by ABYC, and is not equivalent in temperature ratings for the ampacities of given conductor sizes.  NEVER, NEVER use solid core household wire on boats.

Aggregate Electrical Load – Residential Building

“How  much electrical “stuff” can we run “all at once” in our single family residential home?”  This is a key question for both residential applications and boats.  For boaters, it relates directly to discussions about 30A and 50A shore power cords and inlet wiring sizes.

Today, if you have a home of 2000 ft2 or more with an oil or gas-fired furnace, you’ll have a service entrance with at least a 200 amp service capacity.  If your home has electric baseboard heating and/or central air conditioning, it’ll probably have a 400 amp capacity. In the 1960s, we simply didn’t have as much “electrical stuff.”

What does it mean to “have a 400 amp capacity electrical service?”  In a moderate-sized residential building, if the individual capacities of all of the branch circuit breakers in your residential service disconnect panel were added up, there would probably be between 500 and 800 amps of distribution capacity.  For example:

8 – 30 amp double pole breakers for baseboard heating
1 – 50 amp double pole breaker for the range/oven
3 – 20 amp single pole breakers for the dishwasher, washer, and microwave
1 – 30 amp double pole breaker for the clothes dryer
1 – 40 amp double pole breaker for the hot water heater
1 – 40 amp single pole breaker for that great air compressor in the garage
20 or more – 15 or 20 amp single pole breakers for convenience outlets
1 – 50 amp double pole breaker for the air conditioning compressor

Hmmm…   Adds up to 810 amps (+/-) of branch circuit distribution capacity.  Take out the baseboard heating and you still have 570 amps.  However, that service panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at maximum breaker capacities.  Remember, breakers protect wires, so the individual breaker capacity is to protect the wire, not the attachment.  What happens if you exceed the capacity of the 200A/400A main breaker?  Well, in that case you’d blow the main breaker, but without blowing any of the individual branch circuit breakers.  Hmmm…

So to the question, “what does it mean to have a 200A or 400A electrical service?”  A “200 amp service” means that the installed utility-owned drop from the street, the conductors of the ”Type SE,” 3-wire service entrance cable to the electric meter housing, the conductors from the electric meter to the service disconnect panel, the service disconnect panel itself, and the earth ground connection are all sized and designed to operate in a safe manner when handling up to 200 amps for a 200A service, or up to 400A for a 400A service. If you exceed that capacity, that set of essentially unprotected electrical components may fail.  In effect, 200A/400A is the “ampacity” of the unfused and unprotected service entrance feed components.  So even though you have 500 to 800 amps of branch circuit load attachments, if you never exceed a combined aggregate load of 200 total amps, the distribution box will serve you just fine.  If every you do blow the main 200A/400A breaker in the home, have the cause determined by a qualified electrical professional!

Aggregate Electrical Load – Boat

The previous analysis of loading a residence main disconnect panel applies in exactly the same way to boats.  Most cruising-sized boats with 30A shore power will have well in excess of 30A of branch circuit capacity; likewise, boats with 50A shore power will have proportionally more branch circuit capacity.  That power is delivered onto the boat through a (30A)(50A) onboard main disconnect breaker, or compatible ELCI.  As with the residence case, the service distribution panel is not expected to run all of the branch circuits at the same time, nor is it expected that branch circuits will actually run at their maximum breaker capacities.  If you exceed the maximum main breaker capacity, you blow either the main disconnect breaker, or the Shore Power pedestal breaker, generally without blowing any of the individual branch circuit breakers.

The ABYC requires an AC Main Disconnect Circuit Breaker within 10 feet of the shore power inlet.  Nothing is allowed to be connected ahead of that main disconnect breaker except the actual shore power inlet connector.  Recall, the purpose of circuit breakers is to protect wiring, and in particular, wiring hidden from view, and away from reasonably easy access, and running through spaces containing combustable materials.  The AC Main Disconnect Breaker protects the boat’s main inlet wiring (the boat’s “service entrance cable,” if you will) up to the main distribution panel that serves the boat’s individual branch circuits. Remember, the ampere rating of the disconnect breaker must be matched to the ampacity of the wiring between the power inlet plug and the main disconnect panel on the boat.  In the case of boats, the wiring installed by the boat manufacturer should reflect what the naval architect sped’ed for the boat.  Remember, the wires we’re talking about provide power to the AC circuit breaker panel of the boat, and carry the total aggregate current load for the whole boat.  Sizing shore power cords smaller than necessary could be dangerous.

AFCI and GFCI-protected Protection

Since 2008, the NEC has constantly extended the AFCI requirement to now include all habitable areas of a home, including kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, laundry areas, and similar places.  Some states have modified these requirements when adopting the NEC as statewide regulatory code (building codes of all kinds are done on a County-by-County basis in Maryland).  Check local building codes before proceeding.

Since 1971, the NEC has continually expanded the coverage requirements for GFCI protection. Today, GFCI protection is required in all “wet” locations in residential buildings, which includes bathrooms, outdoors locations, rooftops, crawl spaces, unfinished basements, kitchen countertop areas, sinks, laundry areas, bathtub/shower stall areas, boathouses, locker rooms, pool areas: you get the idea.  On boats, the ABYC requires GFCI-protected outlets in heads, galley, machinery spaces and everywhere on the weatherdeck.

Should you wish to retrofit AFCI and GFCI-compliance into an older home (a good idea), a reasonable approach is to replace the conventional circuit breakers in the main  disconnect panel or sub-panel that serves affected branch circuits with combination AFCI/GFCI-protective circuit breakers.  That way, all outlets served by that circuit breaker are AFCI-protected and GFCI-protected.  Combination breakers are available from many manufacturers for about $35 – $45 apiece (as of January, 2018).  Discounts are available for volume purchases.   On the boat, physically compatible GFCI-breakers are not generally available, so GFCI-protected outlets are recommended.

GFCI-protected devices do present some unintended consequences.  A common scenario is for boaters to use adapters to enable a 30A or 50A shore power cord to use a standard 15A or 20A, 120V GFCI-protected utility outlet on a dock.  This provides power for a fridge, a battery charger, and maybe a reading lamp, for a night or two.

In the case of deteriorated, cracked insulation on a shore power cord lying in the water, a ground fault current could easily be large enough to trip a GFCI breaker, and that fault would not go away over time.  That condition is a true ground fault.  Not all trips are caused by true faults.  Sometimes, electronic components (capacitors and inductors) within the familiar portable computer “power bricks” can cause “momentary” surge currents that can trip sensitive GFCI protection devices.  Insulation breakdown on blower motors, pumps  and air conditioning compressors, as well as aging hot water heater elements, can cause transient power leaks.  Power spikes on power lines can trip GFCI devices.  All GFCI implementations are exposed to false faults resulting in “nuisance” trips.  When attaching to GFCI-protected outlets, it’s a good idea to set all AC breakers “off” first, then plug in, then turn branch circuits “on” one at a time.

For marinas and boatyards, starting in 2011, the NEC has required ground fault protection on new construction docks (except residential, single family docks until 2017).  These devices are called Equipment Protective Devices (EPD), and are also subject to “nuisance trips.”  To reduce the incidence of nuisance trips, the NEC has adopted two accommodations to lessen the occurrence of false trips on docks.  First, the size of the leakage current – 30mA – that would cause a marine pedestal EPD to trip “off” is greater than (less sensitive than) a 15A/20A GFCI convenience outlet.   Second, the length of time (duration of) the leakage current needs to be present – up to 100mS – has been made longer.  Since 2011, the rollout of these EPD sensors at marinas has been slow, but they are beginning to appear in greater numbers, and all boaters should expect to see EPD protection of marine outlets on docks with increasing frequency over the next few years.  The electrical knowledge and skills found among dock staff are unlikely to resolve problems for those who do experience nuisance trips at a marina.  Particularly on holidays, weekends and off-hours, high-school and college summer help are not likely to be able to assist transient boaters.

“Nuisance trips” may or may not mean you have wiring errors or equipment faults on your boat, but the fact is, many boats do have wiring errors and equipment faults that until recently have been silent and non-symptomatic.  Obviously, “troubleshooting” this scenario could be very complicated and time consuming.  If you have the skills to do it yourself, it’ll cost lots of time.  If you hire a marine electrician to do it for you, it’ll cost lots of money.  Either way, it won’t be easy or inexpensive.  It may well be that you just have older switchgear equipment, like a reverse polarity light with a filament that provides a “leakage path” from “neutral” to “safety ground.”  This is not an unsafe condition, but it will trip some EPD devices.  What is nasty about this is that “your boat is at fault,” and that’s precisely what you’ll get from the marina operator.

Special Situations – Life’s Little Complications

There are two types of three-phase wiring configurations: “wye” (or “star”) and “delta.” Three-phase distribution systems are used in commercial facilities and larger industrial facilities.  Within this category, I include condos, townhouses, strip mall offices, shopping centers, marinas and boatyards.  So, consider for example the case of three-phase distribution systems feeding end-user attachments in a condo or apartment.

In our “single family suburban residence” model, we learned the US standard voltages of a “single phase, center tapped, three wire” service entrance would be 240VAC/120VAC.  For many technical and economic reasons, light commercial and multi-family residential buildings are supplied from a three-phase, wye-connected service.  In a wye configuration, a 4-pole, 4-wire distribution system comprised of  “ϕ-1,” “ϕ-2,” “ϕ-3” and “N” is delivered into the building.  What is finally delivered, in turn, to the individual occupancy units is a 3-pole, 3-wire feeder analogous to the single phase street feed.  It is not, however, derived from the secondary of a single phase transformer.  Rather, it consists of any two of the three phases that came into the building, together with wye’s “N.”  As an example, suite 100 may receive “ϕ-1,” “ϕ-3” and “N,” and suite 102 may receive “ϕ-2,” “ϕ-3” and “N,” and so forth.

In the wye configuration, the voltages delivered to individual occupancy suites are not the standard 240VAC/120VAC.  Between “N” and any of the phases, the suite would see 120VAC. But between the two phases, the suite would see only 208VAC.  This service is written on paper as “208V/120V Y,” to indicate the phase-to-phase voltage, “208V,” the phase-to-neutral voltage, “120V,” and the fact that the configuration is a wye connection, “Y.”  This practice is common enough in the US that household appliances built for 208VAC/120VAC are commonly available in retail outlets for condo and townhouse dwellers.

Fortunately, 240VAC/120VAC appliances connected to 208V/120V Y services will usually work. Many are made to tolerate the lower line-to-line voltage.  The downside is, appliance efficiency may be reduced.  The power available across the “L1” and “L2” lines of phase-to-phase connection service will electrically be only 85% of the power available from the full design voltage.  As boaters, we need to be aware that many marinas are configured in this way.  If a boat has 240VAC appliances aboard – air conditioning, hot water heater, range/oven, washer/dryer, etc. – those appliances will receive “low voltage” if the marina is configured to provide “208V/120V Y.”

The most significant impact might be to 240VAC pump and compressor motors.  With a low voltage on the appliance, efficiency will be compromised, and motor overheating might occur.  Three phase “Y” distribution configurations are common in marina’s.  Boats with one or two, 2-pole, 30A shore power connections would not be affected.  Those connections are 120VAC.  Those with two 30A shore power cords connected to a “Y” adapter into a 50A outlet on a pedestal are also unaffected.  That’s because even though you are bringing the two different phase lines aboard, your boat does not have any 208V/240V appliances, so nothing aboard is affected.  Boats that connect to shore power with 3-pole, 50A shore power cord are potentially affected, as that 3-pole, 4-wire connector provides 240VAC with the expectation that it will be used on the boat.  Without 240V appliances, there is no affect.

The only thing you, as a boat owner/operator, can do to protect your appliances is to measure, with your onboard volt meter(s), the line voltages (2xxVAC/120VAC) provided by the shore power pedestal, each and every time you hook up.  In this way, you will know what the marina is delivering.  I recommend you become meticulous about this.  If you are not receiving 240VAC – if you are receiving only 208VAC – you will have to make decisions about what to do next.  Do not expect the dock hands that help you tie up to know what they have. Some may, but I would assume many would not.  Frankly, even the marina manager may not know.

DC Electricity On Boats

About This Article

This Article discusses DC Electricity concepts and terminology at an introductory level.  There are always discussions on boating bulletin boards relating to DC power systems on boats.  This article is intended to help those with little or no background or training in electrical systems to understand those discussions.  I have included the most important sub-topics related to 12V and 24V “low-voltage” DC power distribution systems encountered by typical cruising boat owners.

Electrical Safety

There is one, and only one, absolute when dealing with electricity.  VIRTUALLY ALL ELECTRICITY CAN BE DANGEROUS TO PROPERTY AND LIFE.  Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.   The large batteries and large banks of batteries found on boats can produce explosive gasses and store enough energy to easily start a large, fatal fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity!  WEAR HEARING PROTECTION when working in noisy environments, with running engines or other loud machinery.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right test equipment and tools for a job…
If you are not sure you know how to use the test equipment and tools you do have…
Well, then, LEAVE IT ALONE until you do!

USE INSULATED TOOLS when working around electricity, and especially around batteries.   Batteries contain enormous amounts of stored energy.  Accidental contact of a metal tool across the terminals of a battery is an emergency situation.  The tool can actually weld to the battery terminals and be both too hot to touch and impossible to remove without external mechanical force. Whenever working around a battery, pre-plan to have a two foot piece of 2”x4” readily available at hand.  If the worst should happen, use the wooden 2”x4” to knock the metal tool away from the battery terminals.  DO NOT TOUCH the tool; assume it will be far too hot to handle with bare hands!  Once this cascade of events has started, the only way to stop it is to break the tool free of the battery terminals.  Otherwise, the battery will get so hot it will melt and may start a fire.

Be very wary of unfamiliar, pungent odors.  Transformers, motors and most electrical and electronic devices that are in the process of failing often heat up and cause insulating or potting materials to give off strong, pungent odors. TURN OFF POWER and use your nose to track down the source.  Treat this as a true emergency.  If you can find the offending device before it bursts into flame, you’re way ahead of the game!  Turning off the power will usually allow the device to cool off.  Do not restart the device!  Excessive over-heating often causes secondary internal damage that you cannot see.

What Is DC Electricity?

DC voltages at their source are characterized by 1) a stable voltage amplitude of 2) unchanging polarity; i.e., the polarity of the voltage between the supply and return terminals never changes.  One battery terminal is considered “positive” and marked with a “+” sign, and one battery terminal is considered “negative” and marked with a “-” sign.  Terminals are either “positive” or “negative” with respect to each other, nit the external world.  The “positive” terminal is positive with respect to the “negative” terminal; the “negative” terminal is negative with respect to the “positive” terminal.  This distinction is important in using a voltmeter to measure voltages.  A DC voltmeter will provide both the amplitude of the voltage that’s present and the polarity of the conductors or components between which the meter is attached.   The amplitude of the voltage can vary somewhat over time, as over the period of time that a battery discharges, but the polarity of that voltage between battery terminals does not change.  This is the fundamental difference between AC and DC electricity, and that difference leads to all of the technical advantages and disadvantages the different electricity technologies offer to users.

Key Electrical Concepts and Terms

The following are some terms regularly used in listserv posts and widely encountered in discussions of electrical systems and circuits.  Boaters will do themselves a great favor by learning these terms and understanding the concepts these terms represent.

  1. Source – Point-of-origin of an electric current.  Typically for DC systems, a battery or bank of batteries.  Electrical sources are “balanced systems,” in that whatever current leaves must return to the source on a one-for-one basis.  If a return path is not available, current cannot flow and useful work cannot be performed.
  2. Load – The components within an electrical system that consume electrical energy to operate; ex: lights, heating elements, motors and electronics.
  3. Circuit – a network of conductors and components carrying electric current from the source to the load, distributing current throughout the load, and returning current to the source from the load.  Circuits are always closed loops that originate AND terminate at the power source.
  4. Supply (or “B+”) – the current-carrying conductor that transports electric current from the source to the load where power is consumed.  In “negative ground” DC systems as required on boats, often called “B+.”
  5. Return (or “B-”) – the current-carrying conductor that returns power from the load back to the source.  In negative-ground DC systems as required by ABYC on boats, often called “B-.”  Analogous the the “neutral” conductor in AC circuits.
  6. Voltage/Volt – the unit of quantification of “Electromotive Force” (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive force is measure across two points in a circuit.  (Volt, millivolts)
  7. Ampere/Amp – the unit of quantification of current flowing through a particular point in an electric circuit.  (Amps, milliamps)
  8. Current – the flow of electrons through a conductor, or the flow of ions through a liquid medium such as salt water; electric current is what performs “work,” i.e., fulfills the purpose of a circuit.  (Ampere; Amp)
  9. Resistance – physical property of all electrically conductive media that acts to retard or impede the flow of electrons through it.  All conductors have resistance. (Ohm)
  10. Conductor (lead, line, cable) – circuit device that transports electric currents.
  11. Ohm’s Law – Mathematical formula that describes the relationships between voltage (V), current (I), resistance (R) and power (P) in a circuit.
  12. Power – the quantification of the amount of “work” that electric current performs in its application.  In purely resistive applications, this will be light or heat.  In turning a motor, this will be the amount of electrical energy consumed in creating torque.  (Watt) (appropriate torque unit)
  13. Ground – a) a universal standard earth reference voltage of “0” volts; b) conversationally, the portion of an electrical circuit to which all other parts are referenced.
  14. Common – any interconnected portions of circuit to which many other parts of an electrical system are also connected.  If reference is specifically to “ground,” this term references a “return” return path shared by many separate portions of an electrical system.  Example: the  positive and return conductors to flybridge nav instruments may be supplied by a “common” B+ power feed conductor (red) and wired with a “common” B- return (yellow) conductor.  Analogies: “ground,” “B+,” “B-” “buss.”  Opposite: “home run.”
  15. Neutral – a non-ground, normally current-carrying return path for electric currents; customarily used in the context of AC circuits.  In DC applications, B- conductors are analogous to the AC neutral.
  16. Fault current – a current flow that follows an abnormal and unexpected path from its source to its return point.
  17. Short circuit – an electrical fault condition resulting from the unintentional connection of a source directly to a return circuit or earth ground.  This unintentional connection often results in the flow of extremely large fault currents. The electrical system should be designed in such a way that fault currents are automatically interrupted by circuit breakers of fuses.  This condition may not cause overload protection devices (circuit breaker) to disconnect the source of power in “ungrounded” systems..
  18. Chase – enclosed spaces in a building or a boat through which wires are run to achieve access to remote locations
  19. Raceway, Conduit, Spiral Wire Wrap, Split Wrap – varieties of supplemental physical enclosure intended to protect electrical conductors from accidental physical damage, excessive ambient temperatures and vibration.
  20. Switchgear – a generic term for all equipment housings in which fuses or circuit breakers and similar disconnecting or switching devices are mounted.  This term is used across the electrical power industry, from generating stations to transformer yards to neighborhood distribution yards to commercial and residential locations.
  21. ABYCAmerican Boat and Yacht Council, Annapolis, MD.  An organization that produces a comprehensive set of safety standards applicable to boats and boat manufacturers, the marine insurance industry, surveyors, attorneys involved in litigation and boat owners.
  22. NECNational Electric Code; United States electrical design standards for Power Generating and Distribution Systems, state, county and community code regulations  and the electrical construction industry.
  23. NFPANational Fire Protection Association; organization that creates and maintains the NEC.

DC Circuits

Fundamental Concept

The essential components of all electrical circuits are:

  1. a source of electrical energy,
  2. a conductor that transports electric current from the energy source to a load,
  3. an electrical load, where useful “work” results when an electric current flows, and
  4. a conductor that transports the electric current back from the load to the energy source.

By definition, an electrical “circuit” must contain all four of the above elements.  All electrical circuits (DC or AC) originate as a pair of electrical terminals that are connected to power-consuming load devices by conductors (wires) of one type or another.  Electric current flows through a circuit.  If a complete electrically-conductive loop is not available from “source” through “return,” an electrical current cannot flow.  Switches, fuses and/or circuit breakers are used to create an incomplete electrical path from the source to the load.

An electric current is the aggregate of millions of migrating electrons (and ions in liquid media).  In DC circuits, it is “convention” to think of the electric current flowing from positive to negative.  This “convention” Is a “working agreement” across all electrical standards bodies, trades and professions.  By mutual agreement, all electrical diagrams of DC circuits and electronics circuits are shown with symbols that assume current flows from positive to negative.  It is a fact of atomic physics that electrons carry a negative electrical charge, so migrate from a more negative place to a more positive place.  As in most “conventional agreements,” as long as the convention is agreed and understood, the pesky facts of atomic physics can be overlooked and left to scientists.

Circuit “Common” Reference(s)

The term “common” applies broadly to circuit elements that are shared among all of the broader network of electrical attachments in a installed electrical system.  The supply buss (“hot, B+”) and the negative return buss (“B-”) are examples of common circuit elements.

Virtually all DC systems encountered by the general public are low-voltage circuits, generally 12-volts, occasionally 24V or 32V.  Examples are 12-volt motor cycle, automobile, light truck, lawn tractor, residential emergency generator, snow thrower or all-terrain vehicle starting batteries, and similar yard and garden devices.  Other low-voltage battery-operated devices include fire/burglar alarms, Uninterrupted Power Supplies (UPS) for computers and data networks, hand-held spot lights, wireless telephone systems and a very wide variety of portable tools.

For applications in the automobile, truck, and outdoor equipment sectors, the return terminal of the battery is typically attached to the metal frame of the vehicle/equipment upon which the  battery is mounted.  The frame is the “common” return path for all sub-circuits.  Electrical components (starter motors, blowers, horns, light sockets, solenoids, sensors, gauges, electronics, etc) have internal electrical return connections that attach to the vehicle’s frame.  The electrical connection is created when the component is bolted to the chassis of the vehicle.  No discrete return conductor is needed because the metal vehicle chassis is the common electrical return path.  This approach simplifies wiring and mechanical design, reduces component design complexity, reduces material and labor cost, and eliminates wiring and connector materials and weight.  The metal frame of a vehicle is perhaps the most obvious place where the term “common” would describe a broadly-shared circuit component.

There are several factors that affect the preceding discussion as it applies to boats:

  1. most small and mid-sized pleasure craft are wired with 12-volt DC systems; 24-volt and 32-volt DC systems are sometimes used;
  2. some medium and large-sized boats have hybrid DC systems of mixed 12V, 24V and 32V systems;
  3. fiberglass (fiberglass reinforced plastic, FRP) does not conduct electricity, so fiberglass boat construction does not provide a functional “chassis,” or “vehicle frame,” return path; and,
  4. electric currents of even the smallest magnitude flowing in metal hulls, metal stringers and/or metal frame members lead to corrosion of the metals, and are always undesirable on boats.

Electrical appliances and utility attachments intended for marine DC applications are designed to have at least two wires; one for the supply of current that originates in the source (B+), and one for the explicit return of that current back into the source (B-).


In all of the preceding discussion, I’ve intentionally referred to the “electrical return path” using that specific term.  In ordinary conversation, the term “ground” is often used to describe the return path of a DC circuit.  This is a technical “liberty” of conversation, since DC return paths are often not actually connected to earth ground.  “Ground” in this context is a term of convenience and convention.  The return path from a low-voltage DC load to its source (B-) is not inherently at zero volts with respect to its surroundings.  A battery held in hand or sitting in a dock cart has two terminals, but neither is referenced to it’s surrounding environment.

Consider a bird sitting on a high-voltage overhead wire in a residential neighborhood.  The wire is at thousands of volts with respect to the earth, and so is the bird’s body and all of it’s little body parts.  But, the bird is safe, the tiny electric currents that make the bird’s heart beat still work, because there is no return path from the bird’s body to enable a disruptive external current to flow.  As soon as the bird flies off, the voltage is gone.  The bird’s body voltage changes, but the bird’s heart still beats normally, and the bird survives, completely unaffected for the contact with that man-made high voltage.

Consider a car, then, that is mounted on rubber tires.  Since rubber is a fairly good insulator, it would be possible for a DC voltage to exist between the earth and the frame of the car.  Normally small, this voltage can be thousands of volts.  Readers who have ever visited or lived in cold climates are undoubtedly familiar with the static shock that can happen when exiting a the car.  That static shock was a blast of high voltage DC caused by the transfer of accumulated charge from the vehicle, through the body, to the earth.  (Well, for purists, electrons flow from the earth, through the body, to the vehicle, but to the shockee, that detail is uninteresting.)  Static electricity and lightening are the same phenomenon, only on a much different scale!  The possibility of static shock is why every gasoline dispensary in the country instructs drivers to remove portable gas cans from cars and place them on the ground before filling them.  Grounding the container disburses any static charge.

It is technically non-trivial to create a reliable earth ground on a car.  Some readers may have seen ground straps dangling from trucks and some cars.  Used mostly on trucks, those straps are intended to protect toll takers and others who might come into contact with the vehicle from static shocks and to provide a safe path to ground static charges.  It is obviously difficult to create a reliable earth ground on a boat, impossible on an airplane.  Historically, earth grounding was not regarded as an important design goal for DC electric circuits.  And of course, experience with low voltage DC equipment generally bears out that assumption.  We get shocks from the build-up of static, but we don’t get shocks when we step off the garden tractor or use the snow blower.  Who among us has never disconnected a car battery when standing on the ground, and that was not a shocking experience.

What this all implies is that, even though the DC return circuit may not actually always be at the electrical potential of earth ground, the DC return circuit in all of our familiar yard equipment, cars and SUVs is referred to in ordinary discussion as the circuit’s  “ground”.  This use of the term “ground” refers to the functional return path ground, not a safety ground.

Safety Ground

As society gained experience with electricity in the early and mid-20th century, it became obvious that there had to be a way to ensure the return path is always at “earth ground” potential in order to  avoid the possibility of personal harm or property damage resulting from accidental contact with electric power.  A safety ground is not required for a circuit to operate correctly, but it does provide other compelling benefits.

Consider a fiberglass boat.  Aboard, there are many parallel DC sub-circuits.  Water pumps, space and nav lighting, nav and entertainment electronics, windlass, thruster, the propulsion engine, etc.  They are all at distances from one another, and the fiberglass frame of the boat is non-conductive.  A safety ground in a DC system (if present) interconnects the external frames and metal cases of equipment, appliances and utility attachments (light switches, outlets, motors, electrical equipment, radios, etc.) to a known point of common potential.  That common point is always the negative terminal of the battery, and under some specific conditions, the water in which the boat floats or the earth itself.  A safety ground is separate from the functional return circuit, and always involves the installation of it’s own individual electrical conductors.

In service, a “safety ground” is never intended to carry current in normal operation.  However, in a circuit containing an electrical fault condition, a safety ground is intended to prevent a personal shock hazard or mitigate property damage risk by ensuring the electrical potential is at earth ground potential.  It is the “safety ground” that provides an emergency path that allows a circuit breaker to function and disconnect power.

Consider, for example, a bow thruster or an anchor windlass.  We would expect to have a battery positive connection to the positive (B+) terminal of the device’s motor solenoid, and a battery negative connection to the negative (B-) terminal of the device’s motor solenoid.  The motor would then be expected to operate correctly with just these two battery connections.   If we also had a separate conductor from the mounting frame of the device to the vessel’s bonding system, that would be considered the “safety ground.”  The thruster would run just fine without the safety ground, but the device could malfunction and place the frame at some non-zero electrical potential.

Vessel Design – “Grounded” vs “Ungrounded”

Designers of DC electrical distribution systems refer to them as either “grounded” or “ungrounded” systems.  The terms “grounded” and “ungrounded” refer to the presence or absence of a safety ground, not the functional return circuit.  A return path of electrons to their source is always required, but that return path is not always referenced to anything else!

There is valid debate among experts as to whether 12-volt, 24-volt and 32-volt boat DC systems can be of the “ungrounded” design or should be of the “grounded” design.  Today, DC grounded systems are not common.  However, new and emerging vessel propulsion systems containing large-horsepower (hP) diesel-driven DC generators and large-horsepower DC motors (systems analogous to diesel-electric train locomotives) are definitely high voltage applications (often between 600VDC and 1000VDC).  Faced with the emerging presence of true medium and high-voltage DC equipment on pleasure craft, this safety ground design choice is now specifically being re-evaluated in the American Boat and Yacht Council’s (ABYC) Electrical Technical Committee.  We await that outcome.

It is to the advantage of boat buyers and all boat owners to understand the low-voltage DC electrical distribution system.  It’s also an obligation of the buyer/owner to understand whether or not a medium or high voltage DC system is also present.  In the majority of fiberglass-hulled boats, it would be unusual to have a separate DC safety grounding circuit installed.  On some boats, nevertheless, one could encounter one of several possibilities.  The electrical system installation on any individual boat depends on:

  1. the prevailing electrical construction standards at the time of OEM fabrication, often related to prevailing standards of the international geography where the boat was built,
  2. how many people may have added to, or otherwise modified, the system over time, and
  3. the electrical skills those individuals who have performed electrical work in the highly specialized marine environment.

The possibilities aboard a vessel include:

  1. no low-voltage DC safety ground at all (most typical today),
  2. partial DC safety grounding on some parts of the system (not recommended; considered technically inadequate), and
  3. full DC safety grounding, vessel-wide.

The ABYC does not require that low-voltage DC distribution systems have a safety ground, but it does make “recommendations” as to how “grounded” and “ungrounded” systems must be interconnected with the vessel’s bonding system.

Polarity  – “Negative Ground” vs “Positive Ground”

Earlier, I pointed out that a battery held in hand or sitting in a dock cart has two terminals, but neither is necessarily referenced to ground.  All that can be said is there is a fixed voltage between the two battery terminals.  Whichever battery terminal is connected to the vehicle frame determines the polarity reference for that DC system.  If the negative battery terminal is connected to the vehicle chassis, the system is considered to be a “negative ground” system.  If the positive terminal is connected to the vehicle frame, the system is considered to be a “positive ground” system.  With the emergence of solid state electronics and economic pressure to reduce manufacturing cost by sharing components across brands, models and manufacturers, the modern automobile industry world-wide (at least since the 1980s) has standardized around negative ground systems.

The ABYC-approved, and by far the most common, DC systems found on pleasure craft in North America are “negative-ground” systems.  On a boat with other than negative-ground DC distribution system, the panels throughout the boat should be clearly marked to identify the manner of connection.  If there is any doubt, always use a voltmeter to confirm the configuration before disconnecting or otherwise making modifications to the system.

Electrical System Topology

Electrical System Schema:

The schema of Sanctuary’s vessel-wide electrical system contains three major divisions.  This diagram is specific to Sanctuary, showing two 30A shore power connections and a fully-integrated but modestly sized inverter/charger.  That said, the overall model generalizes very well to larger electrical systems based on voltage, inlet, inverter/charger and load capacities and configurations.

  1. AC electrical system division of the vessel includes:
      • 120V Shore Power inlet connections
      • AC Generator (Genset)
      • ABYC-compliant Generator Transfer Switch
      • AC Branch Circuit Distribution Panel(s) – (NewMar – House loads; Weems & Plathe – heat pumps, raw water circulator)
      • Galvanic Isolator
  2. DC electrical system division of the vessel includes:
      • Battery Bank
      • Propulsion engine alternator
      • DC Branch Circuit Distribution Panel(s)
      • Individual component attachments (Thrusters, Windlass, Autopilot, Entertainment, Inverter/charger, etc.)
  3. Interface, or Bridging, or Power Conversion division of the vessel includes:
      • Magnum MS2012 Pure Sine Wave Inverter/Charger

General Topology of the Vessel Electrical System:


An Adobe Portable Document Facility (.pdf) version of this drawing is available by clicking this link: 20161019_electrical_system_topology.

Bonding System Design and Evaluation

2/9/2020: Significant edits to better connect concepts and include technique descriptions.

My previous post (Corrosion Article) discussed corrosion of underwater metals caused by various stray electric currents in the water.  In that post, I made passing reference to “bonding,” “bonding conductors,” and to underwater metals “being bonded together.”  This article looks specifically at the bonding system of a boat.  The objective is to provide a basic understanding of why bonding is installed, what it does, and consider the maintenance needs of the bonding system.

As boaters, we are constantly involved in discussions of the design, equipment, materials, techniques and components of the AC and DC divisions of a boat’s electrical system. When those systems fail, there are usually symptoms, anxieties and inconveniences that boaters notice. Although Internet boating discussion lists are filled with electrical topics, only rarely does one see discussion related to a boat’s “bonding system.”

As in other electrical technical areas, “bonding” is an area where there is a body of common concepts and terminology that apply across a wide range of AC and DC situations. Just as in the “Corrosion” topic, the concepts of bonding are consistently the same, but an understanding of context is essential to avoiding confusion.  Experienced electrical practitioners often take shortcuts with context. For the layman, the only way to get past that is to invest some time in understanding the concepts. After that, understanding context gets easier very quickly.

The terms “grounding” and “bonding” are often used interchangeably, but in fact, they are different. Following are definitions with which most experts would agree:

Ground” is the single-point of electrical connection between an electrical sub-system (like a boat) and the physical earth. This connection is made for the purposes of:

  1. providing a lightning discharge path,
  2. providing a path to bleed off static charge,
  3. sub-system voltage stabilization, and
  4. reducing RF interference.

Bonding” is an electrical connection (usually a network of electrical connections) which electrically interconnect metallic housings and device enclosure components. Bonding:

  1. provides a low-resistance path for ground-fault currents to ensure circuit protection devices (circuit breakers) trip,
  2. prevents dangerous “touch-voltages” from appearing on exposed metal surfaces, and
  3. provides a path for galvanic currents and AC and DC stray currents.

NOTE: there are two important reasons to have a bonding system on boats; 1) mitigation of galvanic currents and 2) AC electrical safety.  Sometimes, we hear and read that a bonding system is not needed or desirable, because it actually provides one of the conditions that are NECESSARY in order for corrosion to happen.  But, that is a very narrowly-framed point-of-view that ignores the importance of the bonding system to the safety of the AC Electrical System aboard a boat.

Figure 1 is a simplified topology overview of the three major divisions (AC division, DC division and Bonding division) of the electrical system of a typical cruising boat, whether sail or power, whether slow displacement hull or go-fast sport fish.  It is representative of the great majority of US-manufactured boats. This topology view is consistent with the “model” electrical system upon which the principal ABYC Electrical Standard, E-11, is based (“AC and DC Electrical Systems on Boats,” July, 2003 – 2018, Figure 10).

The ABYC E-11 standard treats a boat’s DC System as the “central-most” division of the electrical system of the boat, to which all other divisions are attached in a peer-to-peer relationship. This seems a reasonable assumption, since AC systems and bonding systems are neither required nor essential on a boat, but the DC system is always needed for engine starting and the operation of bilge pumps, navigation lighting and (usually) sound-signaling device requirements.


Figure 1: The Bonding System on a Typical Fiberglass (FRP) Cruiser.

All of the conductors shown in green in Figure 1 are part of the boat’s “bonding system,” or “bonding network.” That entire network of conductors works together. In typical dockside conversation, the “bonding system” is often thought of as limited to the wiring shown on the right-had side of Figure 1. The usual term applied to the AC portion of the bonding system is the “AC safety ground.” Note, however, that the AC safety ground is an integral part of the overall bonding network of the boat.

In normal operation, all bonding systems are “silent” and “invisible.” When “everything is right,” the bonding system does nothing, and “everything works fine.” Bonding networks are so quiet and invisible that a boat owner might never know if a fault had appeared.

In fact, the primary purpose of the bonding system is to spring into action to protect us when an electrical fault does occur in either the AC or DC system. The only “normally active” purpose of the bonding system is to control corrosion due to DC galvanic currents.

Today we are very fortunate that the reliability of electrical components is very good.  The mathematical probability, confirmed by life experience, is that electrical faults are relatively infrequent. Given that the bonding system comes into play only when there is a fault, it probably won’t actually be needed very often. If the bonding system does have a defect, unless there is another, second fault, there will be no failure symptom or danger to people or pets. Yes, there may be an increased rate of corrosion, often interpreted as “electrical issues in the basin and nothing to worry about.” These are “handled” as a routine maintenance item, but the underlying cause is often not diagnosed or corrected. The bonding system adds complexity to the boat, but can save many headaches, much expense and even heartache for the boat owner if it is intact when needed. Some bonding system faults can create dangerous situations leading to fire, electric shock, loss of property and in the ectreme, loss of life.

The heart of the DC division of the boat electrical system is the battery/battery bank, including all B+ and B- wiring and all subordinate DC device attachment wiring. “B+” is the term for the DC positive feed (+12V, +24V) that originates at the positive post of the boat’s battery. “B-” is the term for the DC negative conductor that returns DC power to the negative post of the battery. In the common lexicon of conversation, the DC return circuit is often referenced as its “ground” conductor. However, the B- conductor in the DC system carries DC current back to the battery, so it is more properly analogous to the “neutral” conductor of the AC division.

Bonding circuits are intended to carry only galvanic and fault currents; never currents that power equipment or attachments. To avoid undesirable voltage drops in the bonding system, and problems with accelerated electrolytic corrosion, no B- connections should ever be made to any part of the bonding system. Such connections are analogous to a “code violation.”

ABYC E-11, Figure 10, shows the “DC Main Negative Buss” as the central collection point for all DC B- return circuits, as well as for the “AC Safety Ground” and the bonding network connections. The boat’s AC Safety Ground and the various branches of the DC bonding system are all connected together at one place, and at one place ONLY: the “DC Main Negative Buss.”

Neither ABYC nor NMMA “require” the installation of DC bonding systems. Bonding systems are “optional.” However, ABYC E-11 does specify requirements for the bonding system if one is installed. Among US boat manufacturers, bonding systems are the “normal” manufacturing practice.

The primary purposes of the bonding system are to:

  1. hold exposed metal parts at to “touch potential” that is safe for people and pets;
  2. provide a low resistance path for fault currents to trip “circuit breakers;”
  3. provide a single point-of-access to protect multiple structural metals of the boat from corrosion, via a sacrificial anode (zinc, aluminum or magnesium, depending on composition of minerals in local waters);
  4. provide a path for certain DC stray currents to safely exit the boat via the AC shore power safety ground;
  5. disperse static electricity formed in high winds and from nearby electrical storms, and
  6. reduce (attenuate) spurious RF electrical “noise” created by on-board equipment (battery chargers, inverters).

Many of the conductors of a “bonding system” are installed in the very hostile environment of the boat’s bilge. The various metal objects tied to the bonding system include:

  1. thruhulls, seachests, sea strainers and packing glands,
  2. rudder “stem iron,” rudders, rudder “shoes” (skegs), tillers and miscellaneous metal support structures of the steering system,
  3. various steering system components (quadrant, cables, metallic hydraulic lines, hydraulic pumps),
  4. trim tabs and thruster systems,
  5. anchoring system components, including all-chain rodes,
  6. exhaust system fittings and ports,
  7. radio counterpoise and static dissipation “ground plates,”
  8. fuel tanks, fuel filling ports and tank vents,
  9. potable water and black water tank access and vent ports,
  10. generator, battery charger and inverter chassis frames,
  11. solar panel and wind generator frames,
  12. handrail and bridge enclosure frames,
  13. heat pump and circulator pump frames,
  14. stove and water heater frames,
  15. refrigeration (compressor) frames,
  16. etc, etc, etc…

In short, lots ‘o stuff.

Figure 2 shows the hull penetrations on a typical trawler (Sanctuary) built with individual thruhulls (without a seachest).

Figure 2: Typical Hull Penetrations on a Boat with Thruhulls and Without a Seachest

The complete collection of all of these metal components are “bonded” – connected together into a single electrical network – as shown in Figure 3.

Figure 3: Typical Bonding System

Figure 3 is only one example of the construction of a bonding system. Other configurations are acceptable. Take particular note of the large gauge conductor shown in orange. That conductor is the “backbone” of the DC portion of the bonding system. That backbone conductor runs the length of the hull. To the backbone are attached all of the green stranded wire pigtails connecting the metal structures of the boat to the backbone. Also note the transom anode (zinc, aluminum or magnesium), which provides primary galvanic protection to all of the metals connected to the bonding system. When the boat is at anchor, away from shore power, it is the transom zinc that is the “ground” connection point. That is, the single point of electrical attachment to the earth, the primary dispersal point for static electricity and lightening and the electrical connection that establishes the “touch potential” for people and pets for the entire electrical system of the boat.

It would not be unusual if a boat’s owner did not know when the bonding network was last tested. It may have been quite some time; perhaps, never, even on older boats. It is possible that weakness(es) are present in the bonding system. I suggest full integrity testing of the bonding system should be done every three to five years.

Most if us have measured the terminal voltage of flashlight batteries many times. We have probably all measured our boat’s 12V (or 24V) lead/acid batteries at some time. Figure 4 reminds us of the very simple task of measuring the terminal voltage of a “AA” battery:


Figure 4: Measuring the Terminal Voltage of a Battery

This “typical” battery is a classic galvanic cell consisting of two “half-cells” (copper and zinc) located in an electrolyte. Since the battery is always seen as a packaged unit, the term “half-cell” is not commonly used except by engineers, battery manufacturers and technicians specializing in corrosion mitigation. The terminal voltage of a “AA” battery is measured with a digital voltmeter. When a load is connected across the battery terminals, current flows to illuminate a flashlight, for example, or power a radio or GPS.

Key concept: batteries are used to provide the voltage needed for circuits.  With batteries, their intended use means there should be a voltage between the positive and negative terminals.  A direct short circuit across a battery is never desirable, as it will dramatically accelerate the rate at which the battery becomes exhausted.  Inside a short circuited battery, the halfcells will become wasted (a form of corrosion) at an extremely fast rate, accompanied by the generation of heat and gasses.  However, in the case of the “accidental” battery created by the electrochemistry of dissimilar metals in seawater, the whole point of the bonding system is to create an electrical short circuit across the various exposed terminals of that “battery.”  Bonding creates a path for electrochemical galvanic currents to circulate.  Bonding holds all of the metal surfaces at the same, safe touch voltage, but in so doing, bonding also ensures the presence of the conditions needed for corrosion to occur.  That is the reason for the presence of the transom anode (zinc, aluminum or magnesium) in the bonding network.  The transom anode serves as a sacrificial metal (sacrificial anode) that protects all of the important, expensive, valuable more noble metals attached to the bonding backbone from corrosion.

For measuring and troubleshooting the bonding system of a boat, a reference “half-cell” is used. The reference cell is external to the bonding system.  The reference cell behaves in a known and predictable way when submerged in sea water. The reference cell becomes one of the halves of a “battery.” The metals attached to the bonding network of the boat become the other half-cell. In use, the reference half-cell is immersed in seawater outside the hull of the boat, and that seawater is the electrolyte of the “battery.”  Note here that “sea water” is a collective term.  The water in which a boat lots can be “fresh,” “brackish” or “ocean” in mineral concentrations.  It is minerals in the water (primarily salt) that affect conductivity of the water.  Since the water is the electrolyte of our corrosive galvanic cell, the voltage measured across the half-cell terminal will vary in different bodies of water and vary in different places on large bodies of water.

A Silver/Silver Chloride half-cell is the best reference cell with sea water (chemical symbol: Ag/AgCl) because it has known and stabile behavior characteristics in that application. That is, the voltage that other metals will produce against a silver/silver chloride half cell are very consistent across a wide range of temperature and electrolyte salinity.

Conceptually, measuring between the Ag/AgCl half-cell and the bonding network of the boat is the same as measuring between the terminals of a conventional “AA” battery. The bonding system and the Ag/AgCL half-cell immersed in sea water, become the “battery” being tested. The DVM measures the “terminal voltage” of that “battery.”

Figure 5 shows the measurement configuration described above:


Figure 5: Measuring the Bonding System with a Ag/AgCl Half-Cell

As a boat owner, there are two ways to proceed with the testing of the bonding system. One is to hire an ABYC-Certified Corrosion Specialist. This analysis is a form of “boat survey,” although is is highly specialized and not all surveyors offer it as a service. Two is for owners to “do it themselves.”  In the DIY case, one must obtain an Ag/AgCl half-cell, available from and other Internet sources at a cost in the range of $140 – $150.  Or, a “Corrosion Meter,” such as is offered by Promariner.

DIYers will begin their testing by connecting the Ag/AgCl half-cell to the negative terminal of the DVM. Then lower the Ag/AgCl half-cell over the side into the water near the hull, to about the level of the boat’s running gear. The half-cell should not rest on the sea bed. The guiding principle here is, if the bonding system is fully intact and functional, all metals connected to the bonding system are expected to be at the same voltage. Probing any of the bonded metals with the DVM should produce the same voltage reading. If different voltages are noted, something is not right, and corrective action is advised.

The bonding system of a boat – whether connected to shore power or not – should produce a reading on the DVM of between -400mV and -1000mV, depending on the mineral composition of the water. Knowing that the bonding system has all of its metal structures tied together, we therefore know all of the readings must be found at the same voltage if the bonding system is intact.

So how does one figure out what a “nominal reading” for any particular locality should be?  There are two ways to get a good approximation of nominal “hull potential:”

  1. A friend who is an ABYC-Certified Corrosion Specialist offers this simple suggestion: “When starting a corrosion survey, I use a pencil zinc and check the potential between it and the Ag/AgCl reference cell.  This gives me an indication of the best reading that I will be able to get for any protected metals on the boat and it also tells me that the DMM is working and that the Ag/AgCl reference cell is behaving.”  This is completely independent of marina power distributions system, fast, easy and safe.
  2. Place the Ag/AgCl reference cell into the water of the boat basin, and probe the ground terminal (make sure to probe the “ground” terminal!) of any nearby pedestal 120V/240V power connector.  This works because the AC Safety Ground on the pedestal is electrically connected to the earth (grounded, earthed) at the service entrance panel of the dock.  Boats floating in the water of any boat basin are referenced to ground through the basin’s conductive water.  (A Galvanic Isolator interrupts this circuit.)  The basin water is the electrolyte through which galvanic currents flow.  Connected in this way, the Ag/AgCl reference cell is one half-cell of a “battery” and the earth connection acts as the other half-cell.  And when the boat’s shore power cord is connected to the pedestal, the bonding system on the boat is held at the level established by the  shore power service ground.  This method works well on docks with good electrical systems, but can be fooled by non-compliant boats.  If using this system, and reading look “suspicious,” revert to method one, above.

Once a good baseline voltage is established, start to evaluate the integrity of the bonding system and any point along the bonding network that is convenient.  Proceed to probe each of the various metal objects found all over the boat; that is, all the stuff previously mentioned [thruhulls, packing glands, sea chests, rudder posts and rudders, steering system components, exhaust fittings, main engine/transmission, Generator frame(s), battery charger/inverter chassis frames, solar panel and wind generator frames, handrail and enclosure frames, heat pump unit chassis frames, fuel tanks, fuel filling ports and tank vents, potable water tanks, thruster systems, black water tank, etc, etc, etc]. The voltage measured by the DVM should be the same as seen at the shore power connection everywhere. If it is not, something is “wrong!”  Note: from a purely “corrosion” point-of-view, only those components immersed in water are affected.  Other bonded metal equipment/components are checked for the purpose of verifying frame-to-frame touch potential safety and ability to trip the disconnect circuit breaker in an electrical fault event.

The last two steps in this analysis are to discover the cause of any inconsistent voltage reading, and then to make appropriate corrections. Some symptoms one might encounter include:

Symptom Possible Cause
Wide variation of voltages between different metal objects.
  1. Boat is not fit with a DC bonding network;
  2. Damage or corrosion to connections within the bonding system.
Most metal objects have consistent voltages except for one or two isolated objects, “here and there.” Loose, corroded, broken or missing bonding connections to the affected metal object(s).
A collection of several metal objects measure one voltage, but that entire collection is different from the baseline voltage. Broken bonding buss somewhere along the length of the backbone.
The baseline voltage is grossly different than expected (-400mV to -1000mV).
  1. Loose, corroded, broken or missing connections to the transom zinc or the shore power ground. Disconnect from shore power, looking for changes and to check the transom zinc by itself;
  2. Overly wasted transom zinc;
  3. Missing shore power ground connection;
  4. B- connection to the bonding system made in error;
  5. Stray DC electrolytic currents in the bonding system.
No reading occurs when the metal object is probed. Bonding connections absent.

(Note: this will only happen with metal objects above the waterline and not in contact with the water.)

Metal Corrosion and Zinc Wasting

2/12/2020: Major update of the section on DC Galvanic Corrosion; minor text edits.
2/15/2020: Correct repeating typo on diagrams; added text to section on what boaters can do about electrolytic fault currents; attempt to simplify descriptions.
5/6/2020: Add approximate concentrations of salts in various types of water.


On the long list of complex technical topics that boat owners face, corrosion of underwater metals is one of the most complicated, potentially most expensive and least well understood.  While it is not possible to ease the complexity or terminology of the topic, I can at least describe several related “stray current” metal corrosion phenomena in this one place.

Some readers may feel this topic is “beyond their pay grade.”  Like it or not, ALL BOATERS have a personal and financial stake in understanding the basics.   At some time in boat ownership, most owners will face one or more corrosion issues.  Even for those for whom the topic is both uninteresting and obscure, all boaters should know how these phenomena are similar and how they are different.  Some familiarity will allow the affected owner to hire the right expert, understand remediation recommendations, and possibly avoid expensive problems in the first place.

AC and DC “stray” electric currents flow in the water.  Not everywhere, but very commonly.  Because these currents flow outside their normal electrical conductors and devices, they are referred to as “stray currents.”  Worst case, all types of fault currents can be present at the same time.  Boaters should consider all electrical currents that flow in the water as a bad thing.


The basic concept in all corrosion is always the same: there is a voltage difference between two or more different metals, or alloys of metal, which are a) connected together electrically and b) immersed in an electrically active liquid.

The three major “stray currents” flowing in water (or in the earth’s crust) are:

  1. AC “ground fault” currents, resulting mostly from wiring errors aboard boats and occasionally from inadequate equipment design, incorrect equipment selection or AC appliance/equipment malfunction,
  2. DC “Galvanic” currents, resulting from the natural behavior of dissimilar metals in mineral-containing ground water, fresh surface water or sea water, and
  3. DC “Electrolytic” currents – a form of DC “ground fault” current – resulting from wiring errors, equipment faults, and improper equipment use.  This is the way electric vats are set up for electroplating, such as in galvanizing chain, but electrolytic currents are very destructive to boats in the water.

While the basic electro-chemical processes and terminology of corrosion are always the same, the cause is always context-specific.  Understanding the context (AC fault current, DC galvanic current, or DC Electrolytic current) is essential to avoiding confusion caused by the shared terminology.

“Electrolytic corrosion,” is  frequently confused with, but very different from, “Galvanic corrosion.”  To repeat, the concepts and terminology are shared and common to both phenomena; it is the “cause and origin” of the driving voltage that is different.  

No matter the terminology, corrosion currents are a silent attack on every boat, and can cost many hundreds or thousands of dollars for those who don’t mount an appropriate and effective defense.


AC fault currents flowing in the water are often dangerous to people, pets and wildlife.

Worst case, AC fault currents can lead to death by “Electric Shock Drowning:”  Children and pets must never swim in a marina’s basin.  Boat owners and professional divers performing in-water maintenance on boats must be alert to the causes and consequences of AC electric currents in the water.

In general, AC fault currents DO NOT deteriorate the underwater metals of boats and do not cause rapid zinc wasting.  

Repeating, In general, AC fault currents DO NOT deteriorate the underwater metals of boats and do not cause rapid zinc wasting.   There is a great deal of technical understanding about environmental AC ground fault currents that comes from the utility and transportation industries (power transmission, buried utilities, pipeline and railroad).   The 60 Hz AC power found across North America changes polarity 120 times per second.   Whatever molecular metal material that might be removed from a metal in one half-cycle is re-deposited in the second-half cycle.  (Cit: “DC Currents in the Bilge – Not AC – Is the Culprit When Metal Fittings Corrode,” Robert Loeser, Seaworthy Magazine, October, 1996).   AC fault currents must be very large before metal corrosion results.   The AC voltages found around pleasure craft docks (less than 600V) DO NOT cause zinc wasting.

Aluminum can be a minor exception.  Aluminum can be damaged by AC stray currents IF the density of the fault current is greater than 40 Amps per square meter of exposed aluminum surface area (40A/M2).  What that means in English is: a relatively high AC fault current in the water will cause erosion to a relatively small chunk of underwater aluminum.  This combination would be unusual, but not impossible, in pleasure craft marinas.  One m2 is equal to about 10.75 ft2, so it would only take 3.7 amps of AC stray current to cause corrosion damage to a 1 ft2 aluminum part.  This is not an extraordinary leakage current, and this amount of leakage current is definitely dangerous to living beings, but 1 ft2 is a small piece of aluminum.  So trim tabs and outdrives may be “relatively” “safe” at levels that would waste aluminum anodes installed for galvanic protection.  On boats without other aluminum parts, aluminum anodes can waste quite rapidly in proportion to the size of a moderate in-water AC fault current.

Readers can find information on testing for AC ground and leakage fault currents in layman’s language on this website.   The reference article helps owners bring their boat into compatibility with National Electric Code (NEC) standards that require ground fault sensors on docks, and it also dovetails well with identifying and eliminating corrosion issues.

Be aware that some in-water AC stray currents can originate from sources on land.  In that case, the fault current will flow on the green AC safety ground wire (a component of the boat’s bonding system), originating in the basin water and flowing back into the shore power infrastructure.  This situation is not caused by a problem on the boat, and in general, is not something a boat owner can fix.  Always report this finding to marina management.


  1. Assess the boat for AC ground fault and leakage fault conditions.
  2. Correct all issues in order to establish a defect-free starting-point baseline.
  3. Consider installing Equipment Leakage Current Interrupter (ELCI) sensors on boat shore power AC service circuits.  (ref: ABYC E-11, 11.11.1 and ELCI Primer)
  4. Where automatic ELCI sensors are not installed, perform frequent manual monitoring of AC shore power cords with a decent-quality clamp-on Ammeter.
  5. Correct any newly discovered issues as soon as they present themselves.


  1. DC galvanic currents are associated with small voltage potentials that are a naturally-occurring characteristic of all metals.  The specific voltage is determined by the atomic structure of the individual metal (or metal alloy).
  2. The Anode is the electrode of an electrochemical cell at which oxidation occurs; the negative terminal of a galvanic cell
  3. The Cathode is the electrode of an electrochemical cell at which reduction occurs; the positive terminal of a galvanic cell
  4. Anodic/Cathodic: terms which relate the polarity of one electrode to another.
  5. A half-cell: either of the polarized components of a battery, either the positive half or the negative half.
  6. A “Galvanic Series” is a list of metals sorted by their naturally-occurring characteristic electro-potentials.  Different “references” can be used for ordering a “galvanic series.”  The best reference for salt water is a silver/silver chloride cell.
  7. A “galvanic couple” is any combination of two or more dissimilar metals or metal alloys connected together electrically and immersed in an electrolyte.
  8. An “electrolyte” is an electrically conductive liquid (generally) medium.
  9. Dry Corrosion” is the direct attack on a metal by dry gasses (air, oxygen) through chemical reactions which result in surface oxidation.
  10. Wet Corrosion” is the direct attack on a metal by an aqueous solution (strong or dilute, acidic or alkaline) through electro-chemical reaction.  Moisture and oxygen can act by themselves.


The underwater metal alloys on a boat together with the minerals in the surface water in which the boat is floating create the elements of a “galvanic cell” (a battery).  The efficiency and strength of galvanic cells depend on the specific materials involved.  Galvanic currents will always be generated when a boat with dis-similar metals occupies water containing dissolved minerals.  A zinc/copper galvanic couple (common “dry cell” flashlight battery) is a “galvanic cell.”  A “lead/acid” automotive or boat battery (wet cells, AGM or Gel) are examples of “galvanic cells.”

Here are some “typical approximations” of salt and mineral content in various geographic types of waters that will affect the electrical efficiency of a galvanic cell:

  1. Potable Water – water fit for human consumption (generally less than 500 ppm).
  2. Fresh Water – water with total dissolved solids (salts & other minerals) generally less than 1,000 ppm.
  3. Brackish Water – water containing more than 1,000 ppm but less than ocean water.
    • Slightly Brackish Water – water that contains between 1,000 to 3,000 ppm salts and other dissolved minerals.
    • Moderately Brackish Water – water that contains between 3,000 to 10,000 ppm salts and other dissolved minerals.
    • Highly Brackish Water – water that contains more than 10,000 ppm salts and other dissolved minerals, but less than sea water (35,000 ppm).
  4. Sea Water – ocean water with Total Dissolved Solids of 35,000 ppm.
  5. Brine – high salt waste water (generally more than 50,000 ppm).

When the metals making up the galvanic cell (battery) are actually the underwater component parts of a boat (bronze, aluminum, stainless steel), naturally-occurring galvanic currents result in corrosion of some of the underwater metal.   The mineral concentration of sodium, calcium and magnesium salts and many others in the surface water affect the speed at which galvanic corrosion proceeds.

The flow of electrons in a DC galvanic current is always from a more active metal (anode) to a less active metal (cathode) on a Galvanic Series.   All environmental surface water, whether fresh or salt, acts as an electrolyte.  Salt water carries more mineral ions than fresh water, so is more “efficient.”

Galvanic corrosion is a slow process that occurs over many months.  Since it’s the anode in a galvanic cell that dissolves, the point of avoidance/remediation is to artificially force the metal(s) to be protected (relatively more cathodic) compared to a sacrificial metal present in the electrolyte (water).  This is done by adding a “sacrificial anode” made of a very active metal (zinc, aluminum, magnesium) to the mix of less active but more valuable underwater metals on a boat.


  1. All metals have a unique and characteristic electro-chemical electrical potential (voltage) that is a result of their atomic structure.  Elemental metals like aluminum, copper, iron, nickel, and tin have unique electro-chemical voltages.  Alloys of metals like steel, bronze and brass also have unique electro-chemical voltages.
  2. In my article on Bonding System Design and Evaluation, I discuss the concept of “half-cells” of a battery.  A “typical” “AA” flashlight battery is a classic galvanic cell consisting of two “half-cells” (copper and zinc) packaged in an electrolyte.  Since the battery is always seen and used as a complete, packaged unit, the term “half-cell” is not commonly used in lay conversations except by engineers, battery manufacturers and technicians specializing in corrosion mitigation.  However, the concept of “half-cells” is important in understanding corrosion.
  3. There are many classification schemes that can be used to quantify and characterize the electro-chemical voltages of metals.  In marine industries, by far the best and easiest is a silver/silver chloride (chemical symbol Ag/AgCL) “half-cell.”  Silver/Silver Chloride “half-cells” produce a stable, definable, repeatable reference voltage across a wide range of temperature and mineral concentrations when immersed in sea water,
  4. When immersed in sea water (sea water becomes the “electrolyte”), the Ag/AgCl reference electrode is one half-cell of a “battery,” and the metal being measured is the other half-cell.  As in all “batteries,” a voltage will be produced between the positive and negative half-cells, so between the test metal and the Ag/AgCl reference cell, small voltages will be apparent.
  5. When compared to the Ag/AgCl half-cell, different metals and metal alloys produce different, unique voltages.  When sorted into a table according to the voltage measured, the result is called a “Galvanic Series,” or “Table of Nobility.”
  6. A piece of ordinary 316 SS immersed in seawater and measured against an Ag/AgCl reference cell will produce around -50mV (0.0V to -100mV).  Similarly, a piece of silicon bronze immersed in sea water and measure against the Ag/AgCl half cell will produce about -260mV (-260mV to -290mV).  In this example, it can be calculated from the voltages measured between the test samples and the Silver Chloride half-cell, there are approximately 210mV BETWEEN THE TWO METALS.  While this is not a very “powerful” battery, it is enough to cause galvanic currents to flow, and those currents can be slowly and continuously destructive to valuable and expensive underwater metals on a boat.
  7. Stainless Steel (Type 316) is an alloy (iron, nickel, chromium, molybdenum) that is “stainless” because the chromium in the alloy forms a strong, clear coating of chromium oxide.  The chromium oxides needs elemental oxygen from the environment in order to maintain and repair itself.  Elemental oxygen is found in the air we breath, and is dissolved in sea water.  Stainless Steel that has a good chromium oxide coating is known as “Passivated SS.”  Stainless Steel without an effective chromium oxide coating is known as “Activated SS.”  The terms “passivated” and “activated” refer to the electro-chemical voltage of the alloy sample being measured.  I will discuss this further later on.

Perhaps some drawings will help:

Sea_Water_Passivated_SSFigure 1 shows a “galvanic couple” of 316 Stainless Steel and Silicon Bronze immersed in seawater.  The silicon bronze is the anodic alloy, so it erodes due to the natural galvanic voltage between it and it’s cathodic 316 SS couple-mate.  Silicon bronze is an alloy of copper, iron, zinc, silicon, molybdenum and tin.  All of these metals have their own unique electro-potential, but when mixed in a “silicon bronze” alloy, the alloy mix produces about -260vM against the Ag/AgCl reference cell.

In this galvanic couple, the 316SS is the cathodic metal (more positive, more noble).  The Bronze is the anodic metal (more negative, less noble).   The anodic metal will always erode, and in this case, the least noble metal in the bronze alloy is the zinc; that zinc is what erodes away first.  This process is called “dezincification.”  Dezincification is a destructive process because it leaves the bronze alloy physically weakened. As it progresses, the bronze turns a pink color, and is easily damaged and subject to mechanical failure.  This is bad and undesirable if it happens to a $2000 propellor, a thruhull, or a packing gland.

There is a way to avoid this damage:

Passivated_ProtectedFigure 2 shows the same SS/Bronze galvanic couple seen in Figure 1, with an added sacrificial anode of zinc (aluminum and magnesium can also be used as sacrificial anodes).  The more negative electro-chemical voltage of the sacrificial anode (zinc ~ -1000mV) forces the bronze component relatively more cathodic in this overall galvanic couple (that is, more positive relative to the zinc anode), so the bronze component is now protected from corrosion.  The zinc becomes the most active (most anodic, least noble) metal in this new galvanic couple.  By corroding, the zinc sacrifices itself as it acts to protect all of the more “valuable” metals from structural damage.

Ongoing maintenance of sacrificial anodes (whether zinc, aluminum or magnesium) is required to provide continuing protection of the more important components of the couple.  Additional valuable corrosion control techniques include the installation of galvanic isolation devices in shore power ground conductors, cable TV coax ground sheathes, and the ground conductors of (now pretty much obsolete) wired telephone and wired Ethernet connections.  Without devices that mitigate against the flow of galvanic currents, the concentration of salt and dissolved minerals in the environmental water will affect the rate at which protective sacrificial anodes are consumed.  Those in areas flooded by tidal ocean waters and in areas of highly brackish water will replace anodes more frequently that those in lightly brackish or fresh water locations.

Stagnant_Sea_Water_Activated_SSFigure 3 shows the same galvanic couple of 316SS and silicon bronze that we have referenced above.  However, this time, we see the galvanic couple floating in highly stagnant water.  As mentioned earlier, Stainless Steel needs a coating of Chromium Oxide to be, and remain, effectively “stainless.”  Oxide coatings – by definition – need oxygen, and some conditions can cause levels of dissolved oxygen to decrease to too low a concentration to maintain an effective chromium oxide coating.  In that case, the SS morphs from a “passive” (or “passivated”) state to an “active (or “activated”) state.  As noted above, 316SS has an electro-chemical voltage of about -50mV in it’s passivated state, but it has an electro-chemical voltage of about -500mV in its activated state.

This situation becomes a serious corrosion concern in at least two cases.

  1. Some boats do not get a lot of use and can sit for years without moving.  If in the water in an area of stagnant sea water, this can cause the SS to deteriorate, affecting all underwater SS components.  In this somewhat rare circumstance, the SS will become anodic and will erode.
  2. More commonly, boats that are not used are at much greater risk of “single metal” corrosion of SS propellor and rudder shafts.  Single metal corrosion is described later in this article.


The propulsion and genset drive engines on most cruising-sized pleasure craft are fit with two-stage engine cooling systems.  In diesel cooling systems, a coolant (“fresh water”) circulates through the engine block, heads, oil-cooler, turbo-charger, and intercooler.  A heat exchanger transfers waste heat from the fresh water coolant to environmental raw water, where it is eliminated via the raw water exhaust.  Commonly, a second heat exchanger transfers waste heat from transmission fluid into exhausted raw water.

Electro-chemically, the raw water passing through the heat exchanger is an electrolyte.   Heat exchangers contain several different alloys of copper and nickel.  The alloys used in heat exchangers are designed to have galvanic voltage potentials that are close to one another on the salt-water galvanic series.  That greatly slows, but does not stop, the galvanic corrosion which occurs within heat exchangers.  The dissimilar metals of the heat exchanger act as the galvanic couple and the raw water is the electrolyte.

If galvanic corrosion in heat exchangers is allowed to continue uninterrupted, pinpoint leaks will develop in the shell or tubes of the exchanger.  Similarly, pinpoint leaks can develop in raw-water cooled oil coolers, transmission coolers and intercoolers.   The result over time is damage to expensive heat exchangers, as well as the possibility of secondary damage to the engine itself.   Boat owners  must be aware that there are zincs located within the raw water channels in engines and heat exchangers.

Boats with wood, steel and aluminum hulls require special anti-corrosion techniques.  Many sacrificial anodes are required to protect the surface area of metal hulls.  Too many anodes can cause paint to peel from a metal hull, and cause damage to the woods of a wooden hull.  Alternatively, systems such as Electro-Guard ( apply a voltage to a metal hull.   These “Impressed Current Cathodic Protection” (ICCP) systems protect the hull plates and welded joints from galvanic attack by making the hull cathodic to its surrounding environment.   This is one of many areas that are “different” for owners of metal-hulled boats vs hulls of fiberglass reinforced plastic (FRP).


SS, bronze, brass and galvanized steel are metallic alloys that contain several elemental  metals within their compounding mix.   Dissimilar metals within the alloy can experience galvanic corrosion.  “Single metal” corrosion results in micro-fractures in the material’s structure, and often results in surface pitting.  The process can proceed to structural failure.

Anodic and cathodic areas form on the surface of alloys due to surface imperfections in the alloy mix, lack of oxygen and/or other environmental factors.  The anodic areas in the matrix give up electron(s).  The ions left behind form into the visible hydroxyl oxidation residue that is shed.  Corrosion currents flow at the expense of the anodic metal of the circuit, which corrodes continuously.

SS shaft logs and propeller shafts, SS rudders and rudder posts, SS fasteners that attach swim platform brackets to an FRP hull, SS keel bolts, SS exhaust port fasteners, etc, etc, are all candidates for a form of single metal galvanic corrosion called “crevice corrosion.”

In brass that contains more than 15% zinc, like the manganese bronze alloy often used in propellors,  unprotected fittings can undergo a single metal galvanic corrosion process called “dezincafication.”  Zinc within the brass alloy erodes away, leaving behind a weak matrix of copper and small percentages of other metals (such as nickel, chromium, manganese) of the original casting.  What’s left is structurally weak and can fail catastrophically.  “Dezincification” leaves a characteristic “pinkish” color to what once had a golden bronze color; particularly so in broken, exposed areas of a part.

In stainless steel, this process is called “CREVICE CORROSION.”  In aluminum, the analogous process is called “POULTICE CORROSION.”  When stale water lies against stainless steel for long periods or time, the water looses it’s content of dissolved oxygen.  Oxygen-depleted water in prolonged contact with stainless steel promotes crevice corrosion, leading to possible structural failure in stainless steel parts and fittings.  Similarly, water that lies in contact with aluminum for long periods of time promotes poultice corrosion.  Poultice corrosion can result in pinpoint leaks in aluminum fuel tanks.

For thruhulls especially, boaters should use fittings of bronze or Marelon; BRASS FITTINGS SHOULD NEVER BE USED UNDERWATER.

For those interested, I have more details on Galvanic Corrosion and the Galvanic Series for salt water on this website, here:

BoatUS has a good article on electrochemical corrosion on their website, here:

David Pascoe has a good article on electro-chemical corrosion on his website, here:


  1. Install a complete bonding system if one is not currently present.
  2. Install zincs to protect bonded underwater metals.
  3. Perform routine maintenance of zincs on underwater metals: propellor shaft, rudder, and other underwater metal structures.
  4. Maintain the “master” zinc that protects the boat’s bonding system.
  5. Maintain zincs protecting engine and transmission cooling system components.
  6. Use deck fill screw-on covers that are galvanically compatible with under-deck fittings to avoid galvanic corrosion and hidden fuel leaks.
  7. Install an appropriately rated Galvanic Isolator in the shore power safety ground if one is not already present.
  8. Install galvanic isolators to telephone, Ethernet and TV Cable feeds that come onto a boat from shore.


The source of the voltage that drives the process is what distinguishes a “Galvanic current” from an “Electrolytic” current.  Recall that galvanic voltages are a function of the natural atomic electro-potential of the metals of a galvanic couple.  Electrolytic voltages are man-made, not naturally-occurring.  The voltages that drive electrolytic corrosion are often significantly larger than galvanic voltages, and the destructive impact of a DC fault causing electrolytic damage is much faster and more aggressive than galvanic currents.

Screen Shot 2020-02-11 at 15.59.41In Figure 4, the metallic actors (SS and bronze alloys) are the same as shown in Figure 1.  In this case, the elemental voltage polarity of the couple has been reversed by the application of an outside source of DC voltage.  This is a DC fault scenario.  The bronze thruhull in this example will disintegrate, freely giving up it’s copper content into the surrounding sea water.

Electrolytic voltages originate with an externally-supplied DC source; i.e., a battery or its equivalent.  Causes can be a wiring error, chafed/frayed DC conductor, defect or age-deteriorated insulation on a bilge pump B+ wire lying in bilge water, a defect in a DC power supply or a DC generator, a failed piece of DC equipment or misapplication of use of DC equipment.   A common wiring error that can lead to electrolytic currents results from incorrectly wiring the neutral return circuit of a DC device to the boat’s bonding system.  NEVER USE THE DC BONDING SYSTEM FOR THE ELECTRICAL RETURN PATH FOR DC CIRCUITS.

DC electrolytic currents are equivalent to the industrial process called “electroplating.”   In a marina, failed DC equipment can deliver a DC voltage into the basin water.  On a boat, wiring error or a failed piece of equipment can apply a DC fault voltage to the boat’s ground buss.  electrolytic current flows IN ONE DIRECTION through the ground path and into the surrounding basin water.  The anode literally dissolves.  The fault can be on the same boat as the failed equipment, on a neighboring boat or in nearby land based equipment.  Or it can be, simply, in between a source point and a return point.  The fault can be located in shore-side infrastructure wiring, or it can be because of misuse of equipment by a contractor, such as a welder or a DC motor on a marine railway or travel lift.

It is a law of physics that electric current always seeks the path of least-resistance back to their source.  Scenario: imagine three adjacent slips on a dock.  In slip #1 is a boat with a fault and dumping a DC electrolytic fault current into the basin water.  Slip #2 is empty.  In slip #3 is a boat providing a path to ground for the fault current via it’s shore power cord.  So far, only boat #1, the boat with the fault, has a serious corrosion problem.

Now a transient boat comes into slip #2.  The fault current previously passed through slip #2 on the way to ground, but when the transient arrived, that boat’s protective electrical system (Bonding System) becomes inserted into the path of the fault current.  The transient’s bonding system has a lower resistance than the surrounding basin water.  The fault current passes into the transient via one or more underwater metals, passes on through the transient’s bonding system, and exits back into the basin “on the other side” of the transient.  The fitting(s) where the current exits the transient will corrode.  That same electrolytic fault current is now causing damage to boat #1 and boat #2.  Boat #2 is a true victim, safe if plugged into shore power, potentially damaged if not.

All fault currents are always opportunistic.  They simply follow the rules of physics to find the “path of least resistance” back home.

The rate at which metal loss occurs is proportional to the voltage involved and to the many Ohms Law factors that determine the magnitude of accompanying current flow.  At its worst case, this process can sink a boat astonishingly quickly (a matter of hours/days), because with large uni-directional electrolytic currents (electroplating), metal mass can erode away from the anodic terminal(s) very quickly.   The part that gives up metal mass will ultimately suffer structural failure if the process is not interrupted.  If it happens to a thruhull on a boat in the water, the boat will sink.   All underwater metals – propellors, rudders, struts, trim tabs and radio ground planes – can be effectively “dissolved” by these stray DC fault currents.

The best articles I know of for an understanding of this topic are by Capt. David Rifkin, who has good reference articles on his website, here:

Nigel Calder, Ed Sherman of ABYC and Steve d’Antonio have also written about these phenomena,  mostly in fee-based subscription publications like Professional Boatbuilder and Passagmaker Magazine, or in their own for-fee publications.


Like all metal corrosion, zinc wasting is a form of electro-chemical corrosion, always due to DC currents.  Electrical measurements of the basin occupied by the boat would be necessary to determine which mix of stray currents are present at any given time, but zinc wasting is a DC phenomena, and by far most commonly, a galvanic phenomena.

Boat owners can do their own basin-water testing, but it is not a process I would recommend for the electrical layman.   An understanding of the theory of these types of faults, understanding the probes and tools that are necessary, and the skills to correctly interpret test results are necessary.  This can be quite confounding, even to experts.

Boat owners that experience corrosion issues would be better served to hire an ABYC CORROSION-CERTIFIED MARINE ELECTRICIAN.  That said, skilled and knowledgeable boat owners can do their own DC testing with a silver/silver chloride half-cell.  High quality Digital Voltmeters (DVM) can detect AC ground fault currents, DC galvanic currents and DC electrolytic fault currents, but detection and evaluation is highly specific and sensitive to the placement of the measurement electrodes, quality of the test equipment, and conductivity of the surrounding basin water.

By the time a layman has bought the tools, learned to use them, and learned to interpret the results, said layman would be better off financially in hiring a professional who could provide the diagnosis and remediation recommendations as a one-time service.


  1. Diligently avoid having DC wiring wetted or submerged in bilge water.
  2. Never use the boat’s bonding system as a B- “neutral” return circuit for DC attachments.
  3. Avoid facilities (marinas, municipal or private docks, boatyards, etc) where the infrastructure appears to be poorly maintained.
  4. Be alert in marinas located in industrial neighborhoods where ground fault currents from shore sources may be more likely; check with the dockmaster for known issues in the basin.
  5. Avoid facilities with numerous boats that are in a poor state of maintenance and repair.
  6. ESPECIALLY FOR BOATS THAT SPEND LONG PERIODS OF TIME IN ONE MARINA BASIN, the final safeguard against serious stray current corrosion is to employ and depend on a diver that has the experience to recognize this type of corrosion.  Develop a good relationship with the diver and demand at least a verbal report stating explicitly the condition of the bottom, the condition of the anodes and, specifically, any signs of stray current corrosion.  Make the diver a “part of the team,” not just someone who scrapes barnacles.



Oxidation occurs with the release of electrons and the simultaneous shedding of positively charged metal atoms which detach from the surface of the metal.  These particles enter the electrolyte solution as positively charged ions.  Chemically:

Fe → Fe++ + 2e (example with Iron);

Zn → Zn++ + 2e (example with Zinc);

Pb → Pb++ + 2e (example with lead).


Free electrons reach the cathode and react with hydrogen ions in the electrolyte.  Hydrogen bubbles will often form on the cathode; clearly visible in lead/acid batteries.  Chemically:

2H+ + 2e → 2H


If acid is not available, water itself will break down (dissociate) to generate hydrogen ions (H+).  The specific chemistry here depends on the composition of the electrolyte.  Assuming water, water dissociates, forming free hydrogen and hydroxyl ions:

H2O ⇌ H+ + OH

Then, metal ions combine to form metallic oxide, which is the corrosion product:

Fe++ + 2(OH) → Fe(OH)2, or

Zn++ + 2(OH) → Zn(OH)2, or

Pb++ + 2(OH) → Pb(OH)2

AC Electrical System

AC System Overview:

Note: An electrical diagram of Sanctuary’s AC Power Distribution  System as described in this article is located here (Adobe Portable Document Facility (.pdf) file): 20161022_ac_electrical_distribution_system.

The ship is wired to operate from two single-phase, grounded-neutral, 120VAC, 30A shore circuits originating in a dockside shore power system.  Neither the shore grounded (white) neutral conductor (white) nor the ungrounded energized conductor (black) is connected to the safety ground (green) aboard the ship (ABYC E11,

The ship’s AC safety ground (green) and DC negative buss (black) are connected together in the engine room (ABYC E11, and subs).

The ship is fit with a ProMariner® Prosafe-1™ galvanic isolator.  AT the time of it’s installation, the device complied with the then requirements of ABYC A28, 28.13 (now obsolete). The diode pack of the device is installed in series with the ship’s incoming green safety ground wire. The isolator and its control module are located in the ship’s electrical closet. The enumerator/monitor is mounted at the ship’s electrical control center located stbd, in the companionway to the vee berth.

Note: the ProMariner® galvanic isolator control module was disconnected in May, 2016.  The design of the enumerator/control module places a ground fault on the incoming shore power circuit.  That ground fault is used by the device to test the ship’s safety ground wire for continuity.  That ground fault can trip shore power ground fault sensors.  Disconnecting the enumerator does not affect the purpose or operation of the diode pack itself, but it does defeat the self-checking feature of the OEM design of the enumerator/monitor.

The ship is fit with two 120VAC, 30 Amp SmartPlug® marine shore power inlet connections (ABYC E11, and subs). “Shore 1” corresponds to a Newmar® ACDC-1™ “house” AC Load Center (identified with the numeral “1”). AC branch circuit breakers (ABYC E11, and subs) serving the genset battery charger, refrigerator/freezer, 120VAC water heater, inverter/charger and several house utility outlet circuits are installed. “Shore 2” corresponds to a Weems and Plath® “heat pump” Load Center (identified with the numeral “2”). AC branch circuit breakers (ABYC E11, and subs) serving two reverse cycle heat pumps and a raw water circulator pump (“air conditioner” units) are installed.  It is not necessary to connect both shore power circuits in order to use either one; each is completely independent of the other.

The ”AC Master Breaker” on the NewMar® “house” AC Load Center and the ”Master Breaker” on the Weems and Plath® “heat pump” Load Center function as shore power disconnect switches (ABYC E11, and subs and 1). These double-pole breakers isolate their respective AC shore power circuits from the ship’s on-board AC distribution system. The 120V energized current-carrying (“hot”) buss on the AC side of the NewMar® Load Center has been modified from its OEM configuration. Breakers 1-4 on the NewMar® panel are fed by shore line-in or generator power. Breakers 5-8 on the NewMar® panel are fed by the onboard inverter/charger as described later in this document.


  1. NoteThis section of E11 was upgraded to include ELCI in the July, 2012 release of the standard. Sanctuary complies with the July, 2009, release, E11, Sanctuary is not fit with ELCI at this time.

The ship is fit with an ONAN® MDJE™ onboard generator. The generator is powered by aa Onan 2-cylinder, 4-cycle diesel engine. Diesel fuel for the generator engine is drawn from the ship’s onboard fuel tanks. The generator has a 2-stage (fresh water with heat exchanger) cooling system. The generator is rated for continuous operation at 7.5kW, 60Hz. Generator operation (glow plugs, start/stop) is controlled by two rocker switches mounted at the ship’s electrical control center.  The generator and its diesel engine are mechanical devices that require periodic preventive maintenance. Refer to the Onan manual for maintenance schedules.  The generator’s AC output is wired in a 240VAC configuration. The generator starter motor is fed through a BlueSea Systems ML-RBS remotely operated DC disconnect switch (ABYC E11,,

The ship is fit with a Blue Systems® p/n 9093 manual Generator Transfer Switch. The GTS transfers the ship’s AC distribution load centers to either the shore power inlets or the onboard generator. When shore power is available, and the load center disconnect switches are set “on,” the corresponding green “Shore Power” LED on the GTS operator’s panel becomes illuminated. The GTS is of the break-before-make design. This prevents simultaneous cross-connection of incoming shore power source(s) and the ship’s generator (ABYC E11, This allows the generator to be started and run for servicing while the ship is simultaneously connected to energized (live) shore power connections.

Shown following is the wiring layout of the GTS as installed. Shore power service cords feed “Source 1” and “Source 2.” The 240V onboard Generator feeds “Source 3.” The NewMar® AC Load Center for “house” loads is connected as “Load 1;” the Weems & Plath® Load Center for  heat pumps is connected as “Load 2.” The GTS is shown in the “Shore” position. If the generator is “running,” the green “Generator” LED on the GTS operator’s panel becomes illuminated. If both shore and generator power are available at the same time, both sets of LEDs will be illuminated.


The ship is fit with a Magnum® MS2012™ Pure Sine Wave Inverter/Charger.

  1. When either shore or generator power is available, the device operates in “Passthru” mode to forward AC power to utility outlets via circuit breakers 5–8 on the NewMar® AC Load Center and to simultaneously charge the ship’s battery bank. When shore or generator power is not available, the device operates in “Invert” mode as the AC power source for circuits 5-8. The device automatically switches between its “passthru” and “invert” modes as availability of shore or generator power changes.
  2. The neutral buss for branch circuits powered by the inverter/charger is isolated from the neutral buss for branch circuits powered solely by shore or generator power. Magnum requires this separation, which they base on ABYC A31, However, AC neutrals are defined to be “grounded conductors.” Therefore, this ABYC reference seems obscure, since it refers to separation of “ungrounded conductors.”
  3. Configuration of the operational status of the inverter/charger is manually selectable via the ME-RCtm Remote Control mounted at the ship’s electrical control center area.
  4. The “fault” lamp on the ME-RCtm Remote Control indicates a problem that prevents normal operation of the inverter/charger. The two most common faults are spike voltages and out-of-tolerance frequency deviations. These faults sometimes occur when the ship is operating on the generator as its AC power source and heavy loads cycle “on” and “off.” These “faults” can be manually cleared by recycling DC power via the inverter/charger battery disconnect switch in the engine room space.
  5. DC electrical energy to power the inverter/charger originates from either 1) the propulsion engine’s alternator (supplemented by the battery bank), if the ship’s propulsion engine is running, or 2) solely by the ship’s battery bank, if the ship’s propulsion engine is not running. In the absence of shore or generator power, the DC ampere-hour (aHr) capacity of the ship’s battery bank can be conserved by discontinuing use of the Inverter/Charger and its attached AC loads.

Finally, the ship is fit with a 700-watt Xantrex® Modified Sine Wave (MSW) utility Inverter. This inverter is an alternative AC power source that can feed a utility outlet power strip on the ship’s salon nav station. This inverter is a stand-alone device that is not integrated into the ship’s AC distribution system. It is available to power the satellite TV receiver/DVR and the TV. DC energy supply for this inverter is as described above. The device mounted on the aft bulkhead of the ship’s standing closet.

DEFINITION: “Secured State” of the Ship’s AC Electrical System

Aboard Sanctuary, a “secured state” for the AC Electrical System is defined to exist when all of the following conditions exist:

  1. All individual AC house circuit breakers on the NewMar® AC Load Center are in the “off” position,
  2. All individual heat pump circuit breakers on the Weems & Plath® Load Center are in the “off” position,
  3. The “AC Master Breaker” on the Newmar® AC Load Center and the “Master Breaker” on the Weems and Plath® Load Center are both in the “off” position,
  4. The generator transfer switch is in the “off” position,
  5. Shore power service cords are disconnected and stored aboard,
  6. The generator is not running,
  7. DC power to the Xantrex® MSW Inverter is discontinued via it’s disconnect breaker, located in the electrical closet,
  8. DC power to the Magnum® MS2012tm system-integrated Inverter/Charger is discontinue via the DC rotary disconnect switch located in the engine room, stbd bulkhead.

References to ABYC E11 contained in this document:

All references to ABYC E11, AC AND DC ELECTRICAL SYSTEMS ON BOATS, are to the July, 2012, release of the standard unless otherwise noted.